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I don't have this on my homebrew setup, but have you considered continuous capacitive level transmitters? There are sanitary options, and the density and temperature of the liquid don't matter. They are expensive, so you'd have to find one on eBay, like this one...

1 standard cubic foot of propane has 2500 BTU's heating value. A 'standard' CF is at atmospheric pressure and standard temperature. The propane in the bottle is stored in liquid form. Propane has a density of 0.1175 lb/cubic foot, so there are 8.5 SCF in 1 lb of Propane, or 170 SCF in a 20lb...

Something like a swagelok or Parker needle valve would work, it doesn't need to be anything fancy since you're on the low pressure side of the LP regulator. Brass, bronze, stainless, or carbon steel would all work as materials of construction. Here is a typical needle valve, you might find...

Your wiring diagram looks ok to me except the alarm/buzzer doesn't look right. You don't have a wire going to Neutral for that device. The solenoid and igniter looks ok. So you're going to use the PID to cycle the burner on and off?

I think you could make a stepper motor on a needle valve work. The valve only needs a Cv of around 0.11 for 40SCFH at 11"wc (100kBTU at 2500 BTU/scf for propane). I don't know how you would use a level sensor, but I think you could make it work on temperature control of the kettle. That is...