The game has to be balanced first and it has to have strong foundations.
Tweaks could then increase/decrease the difficulty (i.e +1/+2/+3 for highground, +0/+1/+2 ennemie's AC, +0/+1/+2 to allied ST or such things, barrel weight 100%,70%,20%,...).
Even Pathfinder that has A LOT of options to customize your experience don't allow players to custom what is action and what is a bonus action (Don't know the EN words for Pathfinder but you get the idea).
What you ask looks completely unreasonnable and would require to balance the game differently for all difficulty.
I agree, I'd like to see an option to create a "custom" difficulty setting like in Pathfinder. It was great to be able to tailor the diffirent options, instead the usual difficulty presets games have. But I think Larian needs to put more focus on enemy ai, because that impacts difficulty in general (regardless of stat tweaks) and currently e.g. stealth detection is lacking. That was also the issue in Pathfinder; all the tweaks were stats, but even on higher difficulties enemies would usually just "attack closest".
