I've actually considered connecting a motor to a standard pressure regulator for this purpose. I bought some cheap "untested, as is" SMC ITV electro-pneumatic regulators on eBay a while ago but didn't get them to do anything, they're probably broken. I haven't gotten around to opening them up to see if I can spot the problem.
I just checked for the VSO or proportional valves but didn't see much of interest at my price range. I have some oxygen concentrators that use nice little electric flowmeters for automatic flow control but I haven't found anything similar for sale at a reasonable price.
I'm currently using a solenoid valve system similar to your option 3 to provide two different gas flow rates, not pressure. I am considering using solenoid valves and pressure regulators to provide multiple pressure settings for another part of the system in a future upgrade.