Certainly!
This will be a slightly edited excerpt from this homebrew I've been making here, for Fully Playable Actual Dragons- I deemed the only way to make the dragons actually FEEL like dragons was to build them to start at level 10-11, with the first 10 levels representing the features from their race as a dragon- but that in turn meant that I needed to actually start fixing high level play, as well as writing out a guide in my homebrew for other DMs to help guide them if they wanted to allow a dragon PC. (This template is still a work in progress on the balance front, I'd be honoured if you use it and I've been tinkering with it for months, but please don't expect it to be perfect)
https://docs.google.com/document/d/1eFc2qQTY9P3ym9fyMNYHC_mIogpt9kNrOdm5n_yj1cs/edit
So as preamble, the game balance gets pretty awkward from levels 9 to 12 (which is almost certainly why BG3 caps out at level 12), and completely breaks at level 13.
This is for a few reasons, but there are two main stinkers.
AC at higher levels of DnD does not keep up with hit rates. In official content, then the highest AC given to any creature is 25, seen on the Tarrasque and Tiamat. This ignores the fact that three main things can raise hit rates, and only two things raise AC- Magic items, Ability Scores, and Proficiency. Having no way to add proficiency to AC means it becomes badly irrelevant beyond 13th level for most tank builds, and the problem is only made worse by the fact that the game puts magical armour at a truly massive premium compared to magic weapons.
Below, I’ve created a table of levels against the suggested non-boosted AC for an average creature of that threat rating, which doesn’t factor in magic armour, spells, or AC-boosting class features, but DOES assume that opponents will have level-appropriate stats and proficiencies, and that players will be outfitted with level-appropriate magic items (acquiring enough +3 weapons for the entire party at around level 12). AC should be raised by a further 2-4 points for an enemy with high AC, and if it’s a low AC enemy, just use official numbers lol.
Remember, dragons are meant to be high AC, and Challenge Rating is a fucking awful system, so most adult dragons need a bit of an AC buff.
1: 15 (Compare to an ‘optimized’ first level AC tank, a fighter with chain mail, a shield, and Defence Style- they have 19AC, which is within that 2-4 points for a fairly balanced High AC)
2: 15
3: 16
4: 16
5: 17
6: 17
7: 18
8: 18
9: 19
10: 19
11: 20
12: 20
13: 21 (At this point, martial humanoids should have their first +3 weapon, and a hit modifier of about +13, which can easily shred vanilla Tiamat with a little Advantage)
14: 21
15: 22
16: 22 (Raphael from BG3 is here. He has 26AC and more HP than the Tarrasque, and he’s not considered unfair as a threat to a party of level 11 to 12 PCs, just tanky.)
17: 23
18: 23
19: 24
20: 25 (Tiamat should have 28 to 29 AC in order to come across as reasonably durable for the strongest villain in DnD)
To finish this point, DO NOT think that every enemy must have this much AC or a hitrate to match. If your players have characters (dragon or humanoid) who have 21AC, then never be afraid to give an encounter a swarm of weaker foes who have a measly +5 or +6 to hit. Players fucking love getting attacked ten times and being told that 9 out of 10 arrows simply bounce off them harmlessly, and the tenth that hits them doesn’t do that much damage (This can also help with both encounter pacing and the game’s narrative).
Saving Throws get even more broken than AC at higher levels, and are quite possibly the main reason that BG3 and other official content tends to cap out at level 12. If you don’t have proficiency in a DC19 Save, then your chances of passing are horrifically low, even with a pretty solid ability score to draw on- but +2 to the save from a 15 in Wisdom absolutely won’t cut it. This is why Draconic Indomitability (which is pretty much the same as a Fighter’s Indomitable) exists, as most Wisdom and Intelligence Saves at high levels are effectively instant kills if your enemies are smart about using them. And even then, Indomitable is flawed- Advantage is much less effective with a low base chance of success.
For these reasons, I highly suggest that you introduce your own homebrew for your humanoid players designed to allow them to get some bonuses to their Saving Throws- either make a version of Epic Resistance that doesn’t require them to have Draconic Indomitability, or a magic item that gives them a bonus to saving throws (this won’t be quite as necessary if someone is running a high-charisma Paladin)