Really great project and nice collection !
There also are some non-open source but still hackable synth out there, such as the Korg NTS-1. It's not exactly DIY but a "mount it yourself kit" and Korg maintains a C and C++ SDK for oscillo and fx.
Funny part with the x0xb0x, the creator mentioned that the x0xb0x is free as "Free your mind and your ass will follow", the funkiest open source clause
Well, the oscillator is a Schmitt trigger. The trigger waits for a capacitor to discharge pas a hysteresis point, but uses discrete voltage levels to switch. So, maybe semi-analog?
If it helps anyone, bom-squad.com is a very similar project and the whole site behind it is on github as well. Full disclosure I contributed to this project myself.
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.
Sans the Bluetooth, something like the AKAI MPK Mini might fit? Not the greatest of keys, very plasticky, but it is cheap and has pots, keys, pads at least.
Ah, for more a DIY approach, if you have an arduino and/or some basic components laying around, connecting a USB port that can do MIDI (or a DIN MIDI port) together with an Arduino or similar platform and some pots/buttons shouldn't be that hard and also there is a ton of resources online for building just that.
Funny part with the x0xb0x, the creator mentioned that the x0xb0x is free as "Free your mind and your ass will follow", the funkiest open source clause