I played BG2 when it first came out - before IA, before mods, back in the time when people were awed by the red dragon. I quickly developed my own self-imposed restrictions to make the game harder. I believe with the latest edition of IA, that anyone who has never played IA before will probably be unsuccessful without coaching. I also see that those in the club can win on insane! Congrats! So, some suggestions to make your personal games harder follow; choose one or more for the challenge:
• Sequencer-type spells are once per battle (except chain contingency in IA is already limited to once per day).
• Try winning on insane with 5 characters and then 4 (or fewer).
• No IA-forged stuff, except what is in the game already (like ring of protection +2, cloak of protection +2, etc.).
• No IA kits.

Old IA player here and I have been unsuccesful so far. I think first time I played this mod was 2010 so 9 years I have been trying about once per year. Just recently started new game and my goal is to get further than previous time (end of SOA). For me it is mostly matter of patience. I enjoy challenge and developing my skills and creating new tactics and actually see those works (I dont like to read premade tactics and use those, It feel much more better when create own tactics). I have the potential to beat this game, because I am very mechanical min-max max effeciency calculative person. One of my flaws has been to trying to be perfectionist. If I have not been satisfied how battle has went/used too much recources than I would like to use, then I have replanned and tried again. I remember firkraag took me 2 weeks to beat in satisfactory way, and then it really felt good. Other good memories are from improved spellhold to beat 24 grand skels using only 1 char, because had only one +4weapon (R/C low AC, stoneskins and position) and beating Irenicus fight in 2 second before buffs triggered (using multiple sunfire spells using 4 arcane casters), because if fight goes on too long it would be impossible for me to win it.

For many more years this game will provide me enought challenge using core rules. Maybe some day, more challenges.

My own challenge is to do it noreload. I have gotten up to level 13ish party but that is where the tougher disable battles start to kill off my party until finally the protagonist cannot handle the pressure anymore.

I am continuously tweaking my party as well as quest order, but it is not easy to progress on a quick pace with such a challenge. It comes down to being safer now but less powerful later to having a breeze later and suffering now. Problems, problems.

I am sticking to core though. There is naught in insane than extra damage, which I feel is not adding to a tactical challenge per se, and that is why I play the game.