Juggling Characters from Different Series

Although I've finished the Season Avatars series, the characters still call to me with other stories. I've planned for this with a spin-off series that I would start in 2019 or so, after I have a chance to outline the overall arc for the series and individual books in the series. Then I saw submission guidelines for a themed anthology that fits with some elements from the Season Avatars series, and the next thing I know, I'm working on a short story. Even if the story isn't chosen for the anthology, it'll help me establish characters and situations for the spin-off Selathen Avatars series, and I can also self-publish it. However, even though the story seems to be coming together relatively easily, it's taking writing time away from my current novel, Dryads to Discover. I would also like to return to "The Unnumbered World," a short story set in the Catalyst Chronicles universe and featuring Julia Kee as the protagonist. Finally, there's a fourth project, a middle-grade novel called Welcome to Costume City, that I started earlier this year and abandoned partway through the project.

While in one sense juggling these four projects may seem like a matter of setting priorities and time  management, there's also the added factor of what appeals to me. I find (not surprisingly) that it's easier to write characters I'm already familiar with than to develop new ones. However, if I always push new characters aside to spend time with my favorites, they'll never get developed. It seems like my best options are either to work on two projects at once (which is probably better in terms of keeping both stories fresh in my brain) or else speed up my projects so I can get through them faster. I imagine juggling stories and characters only gets harder as you write more stories. Hopefully at some point I'll have enough free time to write them all.