A single encounter involving 15 goblins all with different HPs sounds like a massive headache for the DM and players.
However, having a first combat with goblins all at 7HP, and then (X sessions or levels later) having another combat with goblins all at 10 or 12 HP, representing "elite goblins," is perfectly in line with the rules and an easy change the DM can make for more difficult encounters.
It’s easy to talk about HP and forgetting to talk about the AC 10 instead of 15 and how that damage the spell efficiency balance. Again, this is not about a problem generated by a single homebrew. It’s the problem generated by a combination of them. Elite goblins should have even higher AC by your terms (16 would do just fine)
Oh I totally agree with you here. The problem in BG3 is the double-whammy: Decreasing AC indirectly nerfs saving-throw spells by making attack-roll spells better, and increasing HP directly nerfs all spells. So saving-throw spells are nerfed twice.
I actually think a similar argument holds true for your suggested addition to these "elite goblins": An AC and HP buff preserves the strength of saving-throw spells but now to-hit spells are doubly nerfed (harder to hit and do less proportional damage). To be truly "balanced," enemy saving throws should also be increased. (AC 15->16, HP 7->10, All STs+1)
I think the best combination of simple and balance-preserving is to only adjust either
HP or AC. However, the simple requirement is only relevant for PnP. A video game should use the slightly more complex solution of increasing all 3 enemy parameters.