Adding another difficulty wouldn't really alleviate the vast discrepancy between options that already exist in the game and fix the busted action economy. As the game goes up in difficulty those options will just become more and more mandatory so we're left standing with the same issue we're already dealing with which is that most of the difficulty in this game will have to come from how willing the player is to self-restrict. Tactician in itself is also an increase in difficulty that doesn't make much sense given 5e as a system since it screws around with things like bounded accuracy. Yes, enemies are more of a threat if they can hit you more and if you can hit them less but that doesn't make encounters more strategic and interesting. Larian could just replace every enemy in the game with a blob of goo that has 1000HP and one-shots the player and that would certainly make the game more difficult but I really don't see the point in doing this. The game is really held back about using spellcasters and enemies that utilize CC. Most of the encounters in act 1 and act 2 are basically just comprised of creatures running at your party and performing some variation of a melee or ranged attack. This is also one of the reasons why martials built for pure offense come off a lot better in this game than they do in 5e because they don't have to go up against that many hostile spellcasters that will target their weak saves. A couple of additional enemies here and there, a spellcaster in a group of enemies that didn't use to have one, there is so much that can be done to make any encounter vastly more difficult without touching enemy stats at all. Now obviously, stats are still an issue because the power of an optimized party gets exacerbated by some magic items and feats which means that enemy HP just can't keep up with what the player can put out.
My overarching point is that the balancing effort has to come from multiple directions. Broken stuff available to the player has to be put more in line with the rest of their options, encounters need to be redesigned to utilize more than just enemies hitting you with basic melee/ranged attacks and the stats of enemies need to be adjusted to counteract the general baseline the player is going to have in terms of damage and defenses if they put a decent amount of effort into optimizing their party so even if every party member has a really solid build with decent magic items the player is still required to be strategic about how they approach encounters without being able to bruteforce everything.
