As far as single player goes, it’s actually quite easy for PC users.
If it weren’t for the sheer volume of versions I’d happily set up a repository on Mega for ease of access, but there’s over 30tb of data for all of it, ---- dealing with that.
Googling how to downgrade for PS turned this up, it’s limited to PS4 though.
https://forum.psnprofiles.com/topic/159709-i-made-a-tool-to-easily-downgrade-ps4-games-to-any-older-version/
And the guide on how it works:
php, html, java, C, C++, Visual Basic.
I’m not sure what you mean with cobalt? There’s something on github, but that seems more like a hobby project.