Nice list!
Taking a look at this breadboard circuit by Jordan Aceto:
It may look a good deal more complicated than the SSO. But the stuff on the right is an amplifier plus a bare speaker, standing in for the powered speaker you need to use with the SSO. You could use a powered speaker here and get rid of that half of the circuit (though probably add a couple resistors to divide down the output level). Aside from that it’s one chip, two caps, a pot, and an LDR — hardly more complicated than the SSO. You can forego the LDR but it allows you to vary the pitch by waving your hands or shining a flashlight on it, very cool!
I was present at the workshop this was prepared for, where a bunch of complete novices, many of them teens and pre-teens (teamed with parents) built several of these things successfully.
Thing is, though, it’s not the dead end the SSO is. That same 40106 oscillator is at the core of Moritz Klein’s VCO (linked by @K.ostas above). Added to it is a CV converter so you can play music on it, AC coupling so there’s no DC offset on the outputs, buffering so the outputs can drive downstream modules, and a comparator to give you a pulse wave in addition to the ramp wave output. As noted above you can similarly enhance the SSO, but it ends up not being really any simpler than the Klein VCO, and probably not as good.
I don’t mean to disparage the SSO. It’s good fun. But people are getting the impression it’s something it isn’t, and getting themselves frustrated by trying to turn it into a “simple” synthesizer.