I understand that you don't want players to respec to avoid paying the fine for retaking their oath.
But some limited respec options should still be available. You could just limit Oathbreakers to not changing their class, for example.
I'm at the beginning of Act 3 and realized that I want to change some ability scores, but I can't without retaking the oath and then breaking it again if I want to remain an oathbreaker (which I do). There are no choices in Act 3 that I want to make that would break my oath. I tried reloading an Act 2 save to break my oath, but it didn't work - and maybe for the best since I would have had to replay hours of Act 2 just to change an ability score.
I also can't change any of the spells for my sorcerer multiclass.
Additionally, there are widely reported issues with the Oathbreaker Knight not showing up in Act 3. Currently he's in my Rivington camp, but I don't know whether or not I'll be able to respec if I advance into the Lower City at all. Maybe this bug has been fixed, it's hard to tell, but I wouldn't have to worry about it if I could respec some abilities without him.