I had basically the same problem as OP some years back. Then I wrote down, for each and every task, how much I'd value having already completed it (including how much I'd value having experienced doing it), and how long I estimated it'd take. This immediately gives value / hour. Add some categorization (e.g. to only work on work tasks during the week), and voila: Auto-prioritization. Helped a lot with getting the actually important things done and get reminded to do useful long term stuff when idle time arose.

FWIW, I'm currently rebuilding it as a web-app, if you want to try: https://quuxtodo.com/