proportional gas valve
I looked up the valves and I was not surprised by the astronomic price for them.
Here is a suggestion for a selfmade valve - if you happen to possess a lathe.
If you are experienced you can even make it with a drill press.
The function will be the same as throttle control for model combustion engines, with a needle valve, either being pushed or turned. The result will be a much smaller design than those offered and accurate enough for fine dosing.
And last not least the device will cost you a few Dollars for brass and a solenoid/geared motor.
Concerning safety controlling combustible gases there shouldn't be any problem. There are hundreds of gaskets and O-rings on the market, mostly in stock of plumbers who maintain gas ovens.
Boncuk