I'm not an electrician, but I have risked a Don King hairdo a few times. As I understand it, you can use the double breakers for 120v because in reality every slot in the box is set up for 240v. Those double 120v breakers just use one hot leg for one, and the other hot leg for the other. Since a 240v breaker uses both sides, you can't get a double breaker for them.
In our shops we really don't have 3 phase, the phase converter or VFD just take our single phase power and make the machine think it is 3 phase. Getting real 3 phase power apparently is very expensive.
If the electrical at your house is that bad, it might be worth looking at adding an additional drop from PG&E to power your garage. Not cheap to do, but a side benefit might be the ability to add A/C to your house which might make it an easier sale to the rest of the family.
As far as the VFD, I got excellent service from Wolf Automation. I sent them an email explaining what I was looking for, and a tech responded within a few minutes, and gave me several options at different price points as well as explaining why they were making those recommendations.
https://www.wolfautomation.com/?gclid=EAIaIQobChMI99Owl6Cr6gIVDSCtBh0GOgRCEAAYASAAEgKUtfD_BwE
I installed the VFD on my mill, but I'm still kind of shocked that I was able to get it to work, so not going to even try to suggest a particular VFD. However looking at the 5-7.5hp VFDs at Wolf I see several in the $400-800 range which I suspect will work for you.
