Update: definitely not the heater solenoid. Pulled it out, air tight when closed, fully open when triggered to open. Triggers when it should.
There was a surprisingly small amount of coolant in the hoses coming to it though. I have done maybe 5 or 6 cycles of what you told me, but the fluid level hasn't changed over the past 2.
My remaining suspects: still a stubborn bubble, interior temp sensor, or (shudder) the heater core.