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 think saving throw spells are only nerfed the one time. Martial actions and attack-roll spells would be affected the same by the new hp/ac values yes? If Larian made saving throw spells more likely to hit, would that bring everything back into balance? You'd still have the same amount length in fights, you'd be hitting more to counter the increased hp, therefore concentration spells would be maintained for about the same amount of time. With this more modified system there's more appeal to casual gamers that don't like to miss and it puts more weight on strategy rather than luck. Core rules can be brutal when the dice just decide you should die and your entire party does nothing for 4 rounds in a row. Doesn't happen often, but ooph not fun.
Not exactly. Martial atctions are easier to hit due to lowered AC (buff) but do less proportional damage from increased HP (nerf). BUT, the mechanics of height/backstabbing allow for easy increases to your to-hit chances, another (buff). Net=1 buff
Same for to-hit spells: easier to hit from lowered AC (buff), less proportional damage (nerf), and are benefitted by height/backtabbing (buff) = 1 buff
Saving Throw Spells/ HP spells: less proportional damage (nerf) and no effect from height/lowered AC= 1 net nerf.
The difference between 1 net nerf and 1 net buff is 2 levels of power.
In order to fix things, as you say, Larian needs to make saving throw spells more likely to hit. This would lead to them being still underpowered, but less so.
This would also immediately fix encounters where monsters are unchanged from 5e DMG. It is still easier to hit them using height/backstab, thus ST spells should be buffed.