Majestic Main Burner Not Lighting

[Zrussell94]

I bought a house with a Majestic DVT39 natural gas fireplace.

When we first fired it up it lit just fine. If uses a nest thermostat to control it. We left it off for a couple months till it got colder. About two weeks ago we turned it on and it worked fine, we left the pilot burning but the thermostat turned down so it didn't turn on. We turned the thermostat up this weekend attempting to turn it on and the main burner wouldn't fire. I've done extensive reading and troubleshooting here's where I'm at:
Took it apart and cleaned everything well
The pilot stays lit
Thermopile generates about 350 millivolts
The switch seems to work (millivolts drop about 100 when it is turned on)
After some more cleaning and messing with it, tonight the main burner turned on again randomly but wouldn't do it a second time.

Is it the thermopile? Is there a switch issue? Right now I am leaning towards the gas control valve being bad, but I'm reading in this forum that isn't common.

What do you suggest as my next step? Is it easy enough to replace the gas control valve? Something I'm missing?

Thanks in advance for advice. Pictures attached.