There definitely are serious balance issues that need to be addressed. But many of us are arguing that the problem isn't 5e rules, it's the combo of 5e rules + homebrew decisions that Larian has added. Many of these changes remove much of the tactical decision making from 5e combat, removing synergy from the different classes and turning combat into more of a 'hack and slash'. However, such hack and slash games need to have higher chances to hit to be fun (feel like a superhero) and make up for the loss of tactical thinking. Larian should fully commit to one side or the other.
In the second example, using Lae'zel I sneaked past the guards, came up behind a Goblin still hidden and had a low percentage (25% I think it was) to hit. Predictably I missed. This is why I made the quip about fighting Jedi Masters. Do Goblins have preternatural reflexes? Do they have eyes in the back of their heads? Standing behind someone unnoticed should convey a 100% hit chance so I don't want to hear any more rubbish about tactics because even when you use tactics the game punishes you for it.
You should almost never have only 25% to hit in this game. Lae'zel has a Strength of 17, giving her a +5 to-hit on melee attacks. In order to only have a 25% chance to hit, you need to be attacking an enemy with 15AC and have disadvantage on the attack roll. Lae'zel doesn't have darkvision, so it makes sense that she would be ineffective at fighting while in the dark (mechanically, this would give Disadvantage to hit ~ 25% penalty; you should use lights or characters with darkvision for sneaking in the dark), but being hidden should grant Advantage on the attack to counteract the Disadvantage...
Did you take the Great Weapon Master feat for Lae'zel? If so, this reduces chances to hit by 25% (in exchange for increased damage) which would explain why your to-hit is so low. Turn that off (in your reactions bar) for higher chances to hit.
For Spellcasters the constant missing is compounded by the fact you only get to cast one rank two spell per rest period because not even short rests regenerate that, they don't even fully heal you, they only do a percentage. This makes being a Wizard that you really don't want close enough to be using melee, a total waste of a character slot. This is why every other cRPG in the last twenty years has used a resource like Mana that can be replenished with the use of potions, and have different spells use different percentages. Would that be in line with D&D? Probably not but only your enthusiasts will care, your average gamer will not. Whatever mechanic is used, spellcasters must be able to cast more than one high level spell per rest period, and short rests should regenerate it.
The bottom line is as you say, BG3 combat is not fun, not even close, and this needs to be urgently addressed and the focus of the next patch before any consideration is given to more classes or story.
BG3 combats seem to be balanced around taking a long rest every ~2 fights. Long rest frequently and you'll be able to use your powerful spells more often. At level 3, Wizards have 4 first-level spell slots and 2 second-level spell slots. That's 6 leveled-spells split between 2 combats, which is a fair amount considering that combats last ~4-10 rounds, and this only gets better with higher levels and more spell slots. Warlocks get less spell slots but these
do recharge on short rest.
Also, damaging/buffing spells are almost always better than healing spells in D&D 5e. This is a known feature/issue.