I'm working on a hobby OS. It's mostly just a fun project, but there's certainly a lot of deciding what to write and what to use, and identifying quality libraries when you want to use something. You can start from nothing and build your own board and bring it up with no software you didn't write or you can use a commercially available board, bios or uefi boot, use an existing boot loader, use an existing libc, etc etc, and explore the specific thing you're interested in, while still earning a lot of systems knowledge.

Deciding when to optimize isn't too hard when nobody is ever going to use it (only optimize run time if something really takes too long, otherwise don't do anything tremendously stupid, but optimize for less development time because this is taking forever)

Readable code would be nice, because sometimes it takes a long time to get back to it, but I've also designed in a real reason to come back and retouch everything (x86 -> amd64; aarch64 as a stretch) and maybe I'll make things reasonable then... Although it's not my strongest skill.

Even most simple OSes become complex. Maybe the dependency chain isn't too long though.

Seems to tick all your boxes, IMHO.