I didn't do any math, just going off of experience building quite a lot of encounters here. If an exact calculation is what you want, leave a comment and I'll delete this answer.

Coinflip encounter

This encounter will come down to the initiatives rolled. With 13 AC and 28 hit points, neither of the swarms should live more than one attacking round from the party at level 6, even taking into account resistances. They should however take up approximately that round of attack by the players.

Which leaves the Living Spellbook, which gets a practical one round of free attacks because the players will be dealing with the swarms. If they're careless or unlucky (aka, you felt like giving them a hard time), the book will use two multiattacks on the sourcerer or artificer, who should go down fast. At that point the combat depends on whether they were able to dispel the mage armor. If the swarms got off an attack before dying, I'd say those chances are low. Otherwise they are high and the fight should be a breeze for the barbarian and fighter (the arcane explosion should do approximately zero agains them with a Strength saving throw).

So the 2 scenarios I see happening are:

Swarms have relative high initiative

• The swarms attack the artificer and sourcerer for about 50% of their HP
• The swarms die
• The Living Spellbook effectively takes out the artificer and sourcerer out of the fight
• Mage armor isn't dispelled, your barbarian and fighter have a hard time, but should barely win out before any deadly casualties. Bad dice rolls for the player result in a loss

Swarms have relative low initiative

• Swarms die immediately
• Sourcerer dispels mage armor on the Living Spellbook
• Living spellbook becomes a glass cannon with 14 AC, which both the barbarian and fighter easily break
• Fight is a breeze, biggest risk is a small paper cut by one of the twirling paper sheets

WoTC have released an Unearthed Arcana Wizard subclass called Lore Master. One of the features this subclass gives is:

Spell Secrets

At 2nd level, you master the first in a series of arcane secrets uncovered by your extensive studies.

When you cast a spell with a spell slot and the spell deals acid, cold, fire, force, lightning, necrotic, radiant, or thunder damage, you can substitute that damage type with one other type from that list (you can change only one damage type per casting of a spell). You replace one energy type for another by altering the spell’s formula as you cast it.

When you cast a spell with a spell slot and the spell requires a saving throw, you can change the saving throw from one ability score to another of your choice. Once you change a saving throw in this way, you can’t do so again until you finish a short or long rest.

This seems to cover both effects that you wish your Mage type character to have.

So WoTC appear to be looking at the same sort of concept that you want for your homebrew campaign.

Looking at this trait however it is evident that WoTC, at least, consider the changing of damage type to be not that big of a deal to game balance, but the changing of the type of saving throw to have a potentially greater effect (and thus needed to be restricted).

Taking your fireball example, changing the saving throw from DEX to anything else would eliminate the rogue’s evasion class feature.

In general the saving throws have been sculpted to the type of effect the spell is imposing.

Some salient examples are:

• area of effect spells are largely DEX saves (fireball)
• a spell effecting a targets mental faculties are usually WIS or INT saves (Fear or Feeblemind)

Important Caveat

It obviously comes with the caveat that this subclass hasn’t been playtested as extensively as the subclasses published in the books, and as a result is not necessarily balanced within the game.