Can the average human train their voice to be capable of singing well enough to be ‘decent’? (not pro)

Yes. And moreover more people than not have perfect pitch when singing from memory https://news.ucsc.edu/2024/08/earworms-music-memory-perfect-pitch.html

The Farcaster formerly known as Kchamp. Books, beats, bodybuilding, bright ideas and beautiful things.

katherinechampagne.com 

There was a video I saw of Ed Sheeran playing his first ever recorded song( or a really old one) just to show the audience how bad he was, from that moment I realized that singing can be “learnt”