Thanks for the phrase "Bonsai Programming". It explains perfectly why after many decades of dev work I still spend time on one particular codebase I started working on in 2005 purely for my own use.
I've done iterations/rewrites of it, in order, with C#, Ruby, Python, Go, Free Pascal, and have been back with C# for the last few variants. Each time the code gets smaller, faster, neater, clearer, more powerful, and better documented.
It will probably never be seen by anyone else (I use it for my own stuff), but I get the same satisfaction after almost two decades of working on it that a gardener might get when pruning and tidying a long-lived and long-loved bush or tree.
Very apt.