To senior devs: How do you manage half a dozen projects at once and decide when to prioritize time and do the small tasks yourself and when to let your team members learn and let them take their time...all while you're working on something exhaustive yourself?

Experience. Lot's of it. Once you had to fix enough bugs in other people's code you will have a feeling for what task you can give others. Building the right knowledge about when to prioritise which project is another thing that is just plain trial and error.

In the end everyone has to learn things their own way.

If you wanna have someone else that can maintaine your projects you need to accept that they will fuck up a lot in the beginning but if you keep calm and give them time and good advise they will learn and be a real asset