Also when you see weapon ineffective in the chat log, how the hell do you find out why they're ineffective? Like it's pretty annoying

Simply put-- you can't. Luckily there isn't a whole lot of reasons why a weapon could be ineffective.

the first, most common reason, is simply that your weapons enchantment might not be powerful enough to damage the type of foe you are facing. The "+1" ect. After a weapons name denotes how powerful that enchantment is, so you can try using a weapon with a bigger +n.

The next most common reason in these games comes up exclusively when fighting trolls (or also revevants in icewind dale: trials of the luremaster). To kill a troll you need to hit them with fire or acid damage, so after you bring them to zero health they just fall down and become immune to all non-acid/non-fire damage (because they have no more health to lose). In this case you don't need to be attacking them except with fire or acid attacks.

The last situation comes from enemies using high power protection spells, and actually usually could fall under the first category but might be impractical to do so. Its a good idea to keep a few buff removing spells on hand like "breach" so that you can lower an enemu wizards defenses at just the right moment.

In your original post you mention that your theif is kind of useless in battle. That's not because of your build-- that's just how rogues are in D&D. They're a much better utility class than combat class. And because their capacity to learn their utility far exceeds the challenges they face (with regards to traps, locks, stealing and sneaking) usually people multi or dual class a thief in these games to make them more combat viable.

If you do want to try to make your pure thief combatty you sort of can, but at best he'll be a sort of "floater" who can do both ranged and melee combat okayish but isn't really good at either for 85% of the game. Just try to give him as many Thac0 improving items as possible.

In Throne of Bhaal thiefs get a high level ability called "use any item", at which point they become very combat viable with the help of wands and scrolls.

Also its worth noting that in 2nd edition DND (the ruleset these games are modeled after) back stabs are extremely powerful, but very unlikely to happen for two reasons

First is that thiefs (except shadow dancers) can't hide without being hidden. That is they have to slip away then hide and come back or use some sort of invisibility magic to become hidden.

Second is that thiefs have such poor thac0 that even if they do hide they may miss the enemy so many times they get seen and have to try again.

Personally I found the most combat-viable non-multi/non-dual thief to be shadow dancer, but even so those parties would have been more combat-powerful without a rogue. For team comp you only need a few levels of thief to succeed (imoen is a sufficiently capable thief for 99.9% of the thief oriented challenges in the game.

Good luck!