abandon all hope and do not even think about starting working on a cpu without the full register documentation (which is often not available without an NDA)
pick one of the three big ones (raspberrypi, arduino, esp*)
I'd suggest picking all three of those. Each has advantages and disadvantages for different types of project, and experience with all three will definitely be a good thing.
My suggestion is think up or choose a project you think might be fun, then work out (or ask for advice) on which of those three choices is most suitable, and go learn how to build it.
My suggestion is think up or choose a project you think might be fun, then work out (or ask for advice) on which of those three choices is most suitable, and go learn how to build it.