With this long-term maintaining of projects, strict automatic backups are essential, as are designated archive filesystems where things may reside gathering dust for decades. (One particular file which I dug up a few months back is dated 'October 1984'.)
Version control is mainly 'git', though I will sometimes also take snapshot of a work-tree and work on a fork of that if I believe that a temporary side-development may become part of the main-line in time.
Being a single operative, I can work on several projects in the course of a day simply by selecting which one to work on as the muse strikes.
I don't think my way is much different from most other peoples', though of course we all feel that only ours is the 'one, true, way".