All D20, but DnD 5 especially, are designed and balanced assuming a 4 or 5 player party. And you can kinda tell, when the rules start asking you to apply multipliers to bigger parties, rather than giving any concrete guidelines. Assuming distributed player competence, a 6 person party isn't simply 20% stronger than a 5 player party. It's much higher. There's an entire additional PC, with their own suite of abilities, magic items, and most importantly, actions. Never underestimate the power of having more standard actions than the other guys (unless you're a pack of CR1/2 minions going up against a bunch of level 5 adventures, then you're screwed either way).
In short, designing challenging encounters for big parties is one of the more substantial challenges a GM may have to face. There seems to be a razor thin design space between "no actual danger" and "guaranteed TPK" when planning for large parties. My own personal strategy is "5 players per party max unless you have a super good reason that a sixth needs to be in this particular game, and never ever ever ever more than that (and preferably not 6 for long)," but that probably won't help you, specifically.
First of all, stop giving them one big thing to focus on. There are a couple iconic encounters that tend to necessitate one big monster against a party of intrepid heroes, like dragon slaying. The problem here with big parties is 1 creature generally can only attack one thing at a time, so even if the beastie is downing 1 PC a round, the rest of the party can pop cool downs and beat the timer. If you design the combat space so the beastie can use it's AOE abilites to good effect, you often find yourself looking at a TPK. Quantity may be a quality all it's own, but it isn't everything. Basically, when the party is that big, the single monster encounter HAS to be able to one round KO any given PC, or it's not a threat. And while it's killing one PC a round, the remaining party members HAVE to have a DPR high enough to whittle the beastie down before it kills them all, or they all just die. SO! Anytime you're tempted to let 7 or 8 dungeon crawlers dogpile one big boss monster, resist the urge. Instead of fighting one wyrm, why not a mated pair of adults, maybe with a wyrmling thrown in to harass the squishies? (Actually, this particular piece of design advice sort of holds true for any size party if there's more than one healer to keep the front liners standing)
Secondly (related to the first), if your party outnumbers the monsters, they'll probably win unless each monster is SIGNIFICANTLY stronger than the PC. I can't give concrete CR equivalencies because it's different at different power bands, but 7 lvl 5 PCs who know which end of a longsword is pointy should mop the floor with 5 CR7-8 monsters. The power of two players worth of extra actions is too substantial to ignore. Design the encounter using the guidelines in the DMG, then add a few support casters or bowmen(or bowwomen, or bowgoblins, or bowwhatevers) at a little under CR to bring the numbers within 1 of the party.
Finally, if you're going to do mob (mob in this context being a large angry group, not a MMO enemy, nor a crime family) encounters, consider looking at the minion style monsters from 4th edition. They had decent defenses and attack bonuses, but 1 HP and very low damage. Custom brew up something mob-able, maybe give them advantage for being adjacent to allies, with a decent attack bonus and a beefy AC, but a damage range of a d3 and very low health. And remember, mob fights aren't typically meant to be challenging in and of themselves, they should be hard enough to drain some resources while letting the players feel like badasses for steamrolling through a pile of enemies. Remember, damage spread to 7 players hurts a party much worse than that damage stacked on 1 or 2 frontliners.
First, a caveat.
There are no rules, per se, to tell you how to do what you want to do. You mention the DMG, which does have section on customizing monsters, including ideas like what I suggest here. (It also has a section on building encounters.) However, the systems in the DMG take time and thought, and you're asking for a quicker, dirtier method. So while I would certainly encourage you to internalize the DMG's guidance, what I am presenting here is a method for use on the fly. It is all based on personal experience.
For what it's worth, I have used both of the two approaches I offer here while running, e.g., Adventurer's League modules for tables of anywhere from 2 to 8 players, never knowing exactly how many I'll have.
Option 1: Scale the number of foes.
The single easiest method to scale 5e encounters is to focus on the action economy. In 5e's fine-tuned mechanics, whichever side gets to act more often has a major advantage. That is the reasoning behind, e.g., the legendary actions that some big solo threats in the Monster Manual have -- it's hard to make a solo threat, even a "big" one, feel truly threatening if it is totally outnumbered by the PCs. The very fact that it is outnumbered and unable to act as often as the PCs act means it's likely to get walloped. Legendary actions help to level the playing field.
By the same reasoning, you can make an encounter workable for a variable number of PCs simply adjusting the number of foes they'll face. More foes equals more actions equals more challenge. It's not a foolproof approach; some monsters have abilities that make them disproportionately difficult as their numbers increase. (Quick example off the top of my head: monsters with Pack Tactics get advantage if an ally is within 5 feet of their target, so the more of them there are on the field, the more likely it is they'll have advantage on any given turn.) But this approach does have the advantage of requiring essentially zero additional preparation. If you're writing an encounter with, say, zombies, you're only preparing zombies whether it's 2 or 5 or 20.
A very general rule of thumb is to pit the PCs against an equal number of foes, adding or subtracting 1 or 2 if the foes are very weak or very strong, respectively.
Option 2: Scale defenses, particularly AC, saves, and HP.
Sometimes adjusting the number of foes isn't feasible or desirable. Maybe you have in mind an encounter against a big solo monster or some specific named NPC. In that circumstance, your best bet is to scale defenses.
Combat is, in a sense, a zero-sum game. The longer it takes you to defeat a foe, the more chances (i.e., turns) that foe has to defeat you first. The PCs' offensive capabilities are generally a known quantity in a given encounter. You'll know roughly how much damage the raging barbarian's axe or the wizard's fireball is likely to do. By tweaking their foe's defenses upward, you'll buy the baddies a little additional time to be relevant before they're beaten.
However, be careful not to adjust those defense figures too much. The "bounded accuracy" model underpinning 5e's mechanics makes a +1 to AC meaningful. I have found that adjusting HP by 10-20% per extra/missing PC is generally workable. It is enough to make the difference palpable but not overwhelming. YMMV, of course.
You might be tempted to also adjust the foe's offenses upward. Unless you are really trying to either clobber or coddle your PCs, resist that temptation. Monster CRs are balanced such that raw damage output is matched to PCs of a given level. In other words, if you double the damage your zombies are doing, there's a good chance you could one-shot a PC, or two, or the whole party. If you halve the damage the zombies can do, your players are likely to breeze through the encounter. It is safer simply to increase a foe's defenses and thereby give it more opportunities to hit.
If you are feeling a little creative, you can even add some flavor to represent the adjustments you've made. Those zombies might be wearing scraps of armor (reflecting a better AC), or might be especially old, decomposed, and unsteady (reflecting worse saves and/or HP).
Best Answer
First, a minor point. As a general rule, monsters have Mythic Ranks(MR), non-monsters have Mythic Tiers(MT). For the purposes of abilities dependent on mythic level, they are treated as equal.
Second, under general rules, a monsters MR is equal to half its original CR. In your example, the wolf's MR is significantly higher than average. The benefits to the wolf are: +4 Natural Armour, 32 HP, may 4 times a day as an immediate action add 1d8 to any d20 result(and do so after the results would normally be known), adds +2 to 2 ability scores(the same one may be taken twice), two mythic feats, and up to 5 mythic abilities.
To calculate CR, take half the MR and add to base CR. As to Dual Initiative, for a wolf or other single attack/no spell-casting creature be likely valued at a 1. For a creature with multiple attacks/spell-casting, it would be a 2. As consideration, a monster with quickened spell casting/like ability, could potentially hit a party with 4 spells a round.
As to mythic breaking the traditional CR, it does. Depending on the party make up, and the abilities taken, it can trivialize encounters that should be upwards of impossible. That said, it is entirely possible for non-mythic PCs to fight CR appropriate mythic creatures.
Especially at low levels, mythic should be used sparingly, and remember a CR3 encounter is going to be a very challenging fight for a party of 4 level 1s.