2) Single action per turn - you can do in most cases just one attack per turn. Also, those are mostly simple actions (sword/bow hit, attack spell), with very few opportunities for tactical decisions or synergy between different actions both for single and multiple characters
I do not think second issue ( single action, no synergy ) would be easy for Larian to address as it is inherent in DnD 5e rules and action set. .
This is the easiest part : just increase the party size so you have 1 more action and 1 more bonus action per round and per character.
About the low %to hit, just try to have advantages and you probably won't ever have 40% anymore.
Larian gave us 2 cheats to have an average of +25% to hit. D&D have many more possibilities but there are useless atm or not implemented yet.
A better implementation of the rules and D&D's actions would lead to more possibilities to have this advantages, and more synergies between characters
About damages, don't forget we're lvl 1-4.
You'll find +1/+2 weapons, we'll be able to attack twice/action, we'll find magical +1dx weapons (or maybe not because of this shitty dipping), we'll have special features to increase our damages,...
Larian already give us cheats to increase our damages : dipping and so many (coating/surface) potions.
What you're asking is a complete rework of the game. This is not reasonable, especially because the solutions to your problems are already in D&D (but not in BG3)
