Hmm well i had joy cons that suffered from drift issues and contact cleaner didnt work on them. It was still under warranty so i got the joysticks replaced. However, now that my warranty ran out i bought spare joysticks (theyre really cheap, like 1-2eur per joystick) so i could replace them myself.
I recently did a dpad mod which required me to disassemble the joycon and it wasnt difficult, did it in half an hour on my first try and it was my first time ever disassembling anything. You just have to be really careful, some of those ribbon cables are really fragile. So if u feel comfortable doing that (in case contact cleaner method doesnt work) go for it. I suppose it depends on the price of the used ones as well. Personally, I would rather pay 10-15 eur/dollars more for brand new ones (if that would indeed be the price difference between used & new in ur case)