Its not so much a skill issue, more so phone repair stores are the scams. they break stuff that wasn't broken in the first place and get you to come back with the that issue or say it was also broken. Source: used to work in the phone repair industry and knew people who did this on the regular and is a standard.
The problem is that most phones don't support ROM's like Graphene or Linage, as people installing custom roms is a relic of years past. Next time you get a phone, think about if you want this option for yourself. Also, do keep in mind that some phones have HW backdoors on them, so even installing a stock rom/graphene does not save you from privacy.
fixing phones is the biggest scam ever. once you open the phone you are always going to have problems with the phone. they are not made to be opened and repaired.
everytime i factory reset my device so much bloat/spy/malware gets installed automagically