Nice summary of the field. You can add Zidware, the guy making Captain Nemo and Circus Maximus soon.

I don't know how I forgot Zidware - you are correct.
Zidware - probably the first boutique pinball company out of the batch in this thread. He has 13 Magic Girls and 200 Zombie Adventures on the go. He is a top designer and artist but possibly because he doesn't have the push of management like at Bally / WMS he can take more time. Being a perfectionist is probably his negative as he has lots of support for his high end pins (crowdfunded in effect) but taking a long time and some are getting upset when he starts working on say Kiss.

Captain Nemo - to be honest I don't know much about them so not sure on there start up model

Circus Maximus - had to look it up to learn a bit more and it seems they are doing a Heighway / Spooky model start up.