There are lots of good tips here already. Perhaps I can add one more.
I have about two dozen embedded hobby projects with separate code bases. And in addition to the other tips here about documentation, refactoring, creating libraries of common code (which I highly recommend) I make my projects (code bases) as similar and simple as I possibly can. I use similar architecture, similar code structures, similar naming, similar styling, etc. And I refactor when I revisit a project, as others have suggested.
I value consistency so much that I sometimes do things that wouldn't be the best solution in a single project to maintain overall consistency across all my projects.
Others have mentioned it, but it's worth repeating: you can't eliminate time required to reorient yourself when you need to work on a code base you haven't seen in a while. But you can use use these tips to minimize it.
Finally, there's a very good book on this topic called "Code Simplicity". It's 70 pages, you can read it in one sitting, and I think you'll find it extremely helpful. The author has graciously made this book available for free: https://www.codesimplicity.com/book/