I think this is far easier than it is being made out to be. An instrumental piece shouldn’t really matter, play in the key that you used to develop that particular musical movement/tone. If it is a song, choose the key that you are comfortable singing. When you play/sing cover songs, which one(s) seem easiest to sing or sound the best for your vocal abilities? From there learn your chords to a key:
There are tons of chords in a key charts on the interwebs.