I use this mod rather than Increase Party Size.
Recruit any NPC - Transform NPC Into Playable Character - Make Full Custom Character Party - No party limit - Resurrect Dead
https://www.nexusmods.com/baldursgate3/mods/1646I am running a party of six currently. My two extras are Bryanna (the sister of the True Soul killed by the owlbear) and Cyrel (the merchant dodgy paladin in the Tollhouse). Made Bryanna a ranger and Cyrel a wizard. We are currently in Act 2 (the Last Light).
Can't swear on it but I think this mod may make a full custom party doable.
I don't feel there is a noticeable game balance problem as I use several mods which make things 5e/RAW (playing on normal).