The short answer is “No.” The long answer is as follows:
What you’re proposing is basically making the archetype become a higher powered pseudo-caster. So let’s take a look at what the end-game modifications would create.
The monk is essentially a martial class, which means they will do fine in close quarters and, at a base level, can do fine with just Attacking. While not as hardy as say a Fighter, it certainly isn’t as spell-reliant nor squishy as say a Wizard or a Warlock. As is, the Way of the Four Elements Monk could be considered a half-caster, much like a Ranger or Paladin which can eventually gain 5th level spells. However, the upside to the Way of the Four Elements is that they can regain the resources used to cast their spell during a Short Rest and their resource is more flexible.
Your proposal turns the Monk into almost a full caster which will regain their resources on a Short Rest. So let’s make some comparisons:
Compare the Monk to a Ranger and you’ll find that with your changes, the Monk will do about as well as a Ranger when it comes to just martial fighting, but their spellcasting suddenly becomes much more potent with 6th-8th level spells meaning now they’re wildly outclassing the Ranger.
Comparing it to a Warlock, unless that Warlock has specifically built for it, the Monk should be better martially. Not only that, but the main stats utilized for the Monk’s martial and spellcasting abilities will give the added benefit of additional AC which the Warlock has no equivalent of. Finally, the Warlock is the closest in spellcasting resources to the Monk in that they regain their spell slots during a Short Rest, however this is not true of the Mystic Arcanum (6th and higher level spells) they unlock at higher levels which they can only cast once per Long Rest. So overall, I’d say that your proposal make the Four Element monk almost equivalent to a Warlock on a spellcasting basis, though i’m hesitant to really say that considering no class has the ability to cast Firestorm or Earthquake twice per short rest.
Finally compare it to the Wizard. The wizard can also regain some resources on a Short Rest, but they’re limited to spell slots of 5th or lower level. So basically we run back into the problem that the Monk is regaining their uses of these much more powerful spells than a full-on caster would.
While the Way of the Four Elements Monk is indeed written underwhelming, the way I solved this was by actually giving them passive features in addition to what they already have. While this doesn’t emulate an avatar-like character, it does at least make it more fun and viable.
If you want something a little closer to an avatar-like archetype, I supposed you could add a few 6-8th level spells with the stipulation that their cost to cast is (the level of the spell + 2) Ki and that once cast, they can’t be cast again until the monk finishes a Long Rest.
On the other hand, you could also just make a character avatar-like and role play them to fit the role. You could play something like a multiclass Monk/Druid or Monk/other spellcaster. Heck, you could even just play a straight up Druid, Sorcerer, Cleric, or Wizard and role play them as though they were a Monk of sorts.
Additional Information:
A few of the passive abilities I add onto the Way of the Four Elements monk include things such as learning Cantrips from the Druid spell list at 3rd level, being able to make an Unarmed Strike as a Bonus Action whenever casting a Cantrip as an action at 6th level, or being able to make an Unarmed Strike as a Bonus Action whenever casting a spell or using an Elemental Discipline at 17th level. Basically just little things that kind of put them up alongside the Eldritch Knight archetype.
A couple others that may be iffy depending on how high-powered of a game someone wants to run includes abilities to recover Ki, such as recovering 1 Ki whenever reducing a creature to 0 hp. This is just to facilitate the use of Elemental Discipline more often, especially when throwing weaker enemies at the party.
Best Answer
You can find examples of most of these (thus demonstrating the proper way to use them) in the Sage Advice Compendium.
Class feature names
Class features, such as Cunning Action, Wild Shape, Draconic Resilience, Unarmored Defense, Empowered Evocation, etc., should be capitalized (but not otherwise italicized or anything).
Examples:
and
Pretty straightforward.
Feats
Yep, they're capitalized. This can be easily found by looking at the Feats section of the Sage Advice Compendium:
and
Class feature subchoices (e.g. Eldritch Invocation names or Fighting Styles)
Yes, they're also capitalized. An example for Fighting Styles:
And for Eldritch Invocations:
Class and subclass/archetype names
Class names are not capitalized:
and
But subclass names generally are capitalized:
and
and
Note that there is a question where "thief" isn't capitalized:
...And several questions about wizards that refer to them as "[school]-er" instead of "[School of X] wizard":
and
and
That said, though these are referenced in the descriptions of at least some of these subclasses (e.g. the School of Abjuration wizard's), these seem more like in-universe terms used to describe members of the subclass rather than intentional stylings of the official/mechanical name of the subclass.
Abbreviations of ability names (e.g. Dex vs. DEX, for Dexterity)
The Sage Advice Compendium writes out "Dexterity" every time it's used... But the abbreviation does appear elsewhere in the rules text - specifically the Armor and Shields table in the basic rules and in the PHB (p. 145). The AC of leather armor, for instance, is listed in the table as:
And the Strength requirement for chain mail is listed as:
...So the correct format of abbreviating an attribute score seems to be to take just the first 3 letters of the ability and capitalize the first letter.
The abilities themselves (Strength, Dexterity, etc.) always have the first letter capitalized:
Spells, and a note on magic items
Your understanding is correct. This is true for both spells and magic items; they're in all lowercase, except for proper nouns (and unless it's at the beginning of a sentence or something), and they're always italicized:
and
and
An example of one with a proper noun:
Another:
Other stuff you didn't mention
The names of specific actions are capitalized: Dash, Dodge, Use an Object, Cast a Spell, etc. Example:
However, as you can see from the above quote, the classifications of "actions" within the action economy (action, bonus action, and reaction) never have their names capitalized. Only the names of specific actions are capitalized.
This is also true of the names of monster traits and actions, though the monster names themselves are not usually capitalized (unless referring to a particular named individual, such as Strahd von Zarovich):
and
and
Correspondingly, the names of player races aren't capitalized either, though racial traits are:
and
Creature types (which both PCs and monsters/NPCs have) are now capitalized. This was a change made as of the release of The Wild Beyond the Witchlight in late 2021, as explained in the "Creature Evolutions" article on the official D&D website:
(The linked article indicates that subsequent books will continue this practice. Earlier materials may or may not eventually be updated to bring them in line with this practice as well, but this isn't mentioned in the article.)
Magic items are lowercase and italicized, as spells are (as mentioned above):
and
The names of languages (e.g. Common) are capitalized, which helps distinguish them from the name of the race that speaks it (which might be the same word but not capitalized):
Creature sizes are always capitalized:
(More examples can be found in the section of the basic rules on creature size.)
And, of course, book names are always capitalized and italicized, but that's just a regular style guideline.
Official resources
I also came across an official D&D style guide by Wizards of the Coast. There's an older version of the D&D Style Guide document (1.04a) here, but the latest version (1.08a) is available on DMsGuild as part of this free product: DMs Guild Creator Resource - Style Guide Resources. According to the product page, the style guide was last updated in January 2019 (this article summarizes the changes from 1.04a to 1.08a).
In general, it follows the Chicago Manual of Style and the Merriam-Webster's Collegiate Dictionary. It includes some styling details not discussed above. The specific official styling rules for D&D game terms are detailed on pages 6-7 of the PDF.
The package also contains this Forgotten Realms Style Guide (the version in the DMsGuild product seems to be unchanged from this PDF). It mostly provides setting details on tone and background as help for designers, but there are some guidelines provided on styling. For instance, it lists certain terms that aren't used in the Forgotten Realms (e.g. "o'clock"), certain terms and names invented for the setting (e.g. "peeptalk" for the language of birds), and the "correct" adjectival forms for citizens/products of certain locations (e.g. "Evenor" for people/things from Eveningstar).
(The Style Guide Resources product also includes a D&D IP Guide about important aspects of D&D's intellectual property, and a short guide by Chris Perkins & Greg Bilsland on the principles of writing adventures. Neither of these contains any styling guidelines.)