I you want a really accessible diy synth project: I recently did an m8 headless build inside of an anbernic r33s. It's basically lego, though you can do a little soldering if you want to, to connect the teensy directly to the r33s board (I used an external cable). And the m8 is one of the best portable synths out there. Cost me like 60€ total.
> I you want a really accessible diy synth project
On that vein, I just started building my own analog synthesizer from scratch, and found this YouTube series which is very beginner friendly and with lots of explainations even for us who don't have much hardware experience: https://www.youtube.com/watch?v=QBatvo8bCa4
The circuit after part 3 already sounds good enough to be used in real projects, yet it's simple enough to fully understand.