Good man going for the manual option (and good to see you back posting regularly, like your curiosty for learning stuff!). Recall years ago 1st changing and it feeling awkward at first, but you quickly get used to to it mates, so please persist. It then carries over to all other racing games and you won't want to turn back to auto then...
Yes aids certainly can slow down. From what I can glean, the general recommendation is abs full, tcs 0 or 1. 0 generally considered quickest if you know the track and can handle the car, but running zero can be slower in that if you keep spinning out by pushing throttle too hard, you'll obviously lose time v being consistent but power/acceleration being cut slightly by tcs cutting in when accelerating out of turns.
Also worth bearing in mind tcs only kicks in if you do hammer it too much out of bends, so if you drive perfectly and running 1, in theory it may not even be activated. I sometimes ramp up tcs if it rains hard, particularly if I haven't had chance to enter tge pit row to change to inters/wets.
The other aids like asm, counter steer are too intrusive and effect how the car handles and slows it down too much. Although tge odd person mentions car sometimes benefits from them in license or missions, where cr4p handling can't be tuned out. Not only slows you down but becomes a crux potentially.
Basically my opinion formed from reading this forum is yiu want to aim for abs on, tcs 0 or perhaps 1 on some handful cars... rwd muscles, or say highly tuned 787b (I used tcs1 on a menu recently as was tedious having to run gauntlet of it not spinning out on tough daytona vgt series...). Some run abs weak, but that seems a very small % of players mentioning that... I would bit tge bullet, remove ALL aids apart from abs and tcs. Even if it means running 3 or 4 tcs and gradually whittling it down... which you most certainly will do. Pick more stable cars so experience less frustrating, and therefore less likely to get despondent and switch all on aids back on (so avoid tokyo gltch tomohawk, engine swapped m3, high powered rwds muscle cars, perhaps use 4wd or really stable praino tuned car or gr 3 or 4 sector cars with high grips... Alfa 155 is a good handling car.) You've got this 👍.