This is a fascinating project. Almost tempting me to give BG1 another go (I got bogged down in one playthrough and never finished it). What's the most enriching mod for a replay, would you say?
Technically, the tweak mods that add ease of use components (such as ammo stacking) would meet that definition, but it feels like a bit of a cop out to say those.
The ones that I personally find the best are the item randomizer and SCS. Without item randomizer, it's hard to stop yourself thinking "There's a good weapon at location X, I will grab it as soon as possible and use it for the rest of the game". SCS adds challenge, most notably with casters prebuffing, but there is quite a bit of variety between which specific prebuffs are used so they don't feel too samey (though I'm somewhat worried that may change in the SOA portion of the game).
If you've only got one unfinished playthrough under your belt, then I would recommend the NPC mods (as vanilla BG1 NPCs are quite boring when they're in the party). There are a bunch of mods that either add content to the vanillla NPCs or add NPCs with more content to the game.
BG2 has a lot more mods available, and for the BG2 portion of the game I am very much a fan of the
colours of infinity mods (they are completely standalone, with the only point where one of the mods influences the other being a single unimportant line of dialogue in "I Shall Never Forget" changing if you have played "Tales of the deep gardens").
If you found your playthrough of BG1 bogging you down, feel free to abandon it and go for BG2. BG1 replicates low level DnD, and BG2 replicates high level DnD, and they are very different play experiences, no matter what edition you're playing, and one if one of them doesn't float your boat then don't let that be a blocker to enjoying the other one.
