I want my 6 slot full custom party.
And please don't make me run 6 game clients to achieve that again.

There would be nothing stopping anyone from making a 4 slot (or solo) run. I definitely approve and endorse challenge runs.
But right now we are being stopped from having a proper party and it does affect the complexity of encounters and gameplay negatively. I am already bored of random NPCs acting as hit point buffers. Why can't i bring my own ?

Look at it this way: If you removed 2 pieces from a chess board you would remove millions of possible moves. If you remove 1/3 of the pieces the game becomes trivial.
I don't want to play tic-tac-toe. I want there to be serious strategy involved in a TURN BASED GAME mind you.
You should take a page out of the book of the Sword Coast Stratagems mod for BG2 which increased the difficulty by adding missing spells and ability (and thereby moves and strategies) instead of cranking up the damage by 200% Bethesda style.

If you wanna take the crown from BG2 there is no way around a 6 slot party. Period.