I am working my way through Solo GM Guide by Geek Gamers. I have also downloaded: The Solo Role-player - The collected archive.pdf is Kenny Norris’ blog from 2015 to 2016, downloaded when removed in 2017. It is free on DriveThruRPG.
Not that it is definitive, but these are good resources for solo gaming.
My goal is to improve my DM’ing skills while having fun.
I plan to look into Modiphius’ 2d20 system. It seems too crunchy for my taste, but it is worth seeing the other side of the spectrum from Tiny D6.