Is there a detailed guide on how to min/max characters in ToEE/its mods? I've found a few things here and there but nothing in depth. I want tips on when and what to multiclass into, feats to combine and so on.
I think you would have better luck doing a google search for min/maxing PCs in D&D 3.5. There is a lot of information out there. ToEE follows the 3.5 rules quite closely, so a lot of the info will be useful to ToEE. The main limitation is that ToEE does not incorporate a number of different rules and options that are available in the P&P version. You should definitely use the Co8 mod, as it fixed many many bugs, and temple+, as it is steadily expanding the options for PCs in ToEE. More classes, prestige classes, different races to use, more feats, etc etc.
Have a look through the Game Guide & Walkthrough. Lots of info there. https://co8.org/community/index.php...le-of-eight-modpack-new-content-edition.8742/
The most optimal classes are the primary casters - Cleric, Druid, and Wizard. Go Human. Take Improved Initiative and Combat Reflexes. Have a high DEX and casting stat. Take Craft Wondrous and Craft Arms & Armor. Continue from there.