I've got a level 39 solo assassin in chapter 6 at the moment. I've got ToB installed with an XP cap remover. I meant to dual to a fighter in the early thirties, but I just realized that I forgot to. Now, if I dual, I can't use any of my thieving abilities except for my ToB HLAs (so no backstab, poison weapon, hide in shadows, etc). If I do dual, then I won't get back any of my thieving skills till I make 8 million xp in fighter. Is it still worth dualing?

its gonna be a long game in the meantime, and will you regain the levals in time to use them before the end ?

I wouldn't bother

you could also use shadowkeeper or cluaconsole to remove exp to put you back to the early 30s. Then if you dual, you won't have to get quite as many levels before reactivating.

Just realized that I have the level 40 mod installed, not the level 50. So being a thief is practically done. Should I just dual to another class right now so that I could potentially still get back my thieving abilities? Cuz if I go to level 40 and then dual, then I won't ever get back my thieving skills.

if you're going to be locked at level 40... it might mean a TOTAL of 40 levels. In which case you would be forever stuck as a level 1 whatever with 39 thief levels stuck inactive