I just LOVED your pictures and GH heating project, Ryan.
I think the on demand type gas heaters (which have gotten fairly affordable these days) are well suited; their usual durability problems are all associated with open systems and water minerals; for a closed system like yours that's a non-issue and it should perform well for a long time without descaling.
Because of the HUGE losses at night from the glazing, you may have issues air temp issues, (more likely just with gas consumption) though at least you have a fabulous lack of loss to the earth, which by reported data accounts for 1/3-1/2 of the thermal loss besides the upward/outward loss through the glazing for typical GH's.
I'd love to see you tackle the glazing loss via some method(s) such as insulated panels or soap bubbles. It's a difficult task that few have tackled. You clearly have the right stuff for it.
I'm a retired EE/CS guy, and appreciate the technical side of things more than most. I say BRAVO to you and your GH project!
