Much more analogue solution, but I fitted one of these https://www.carbuilder.com/uk/fan-speed-conversion-switch-three-position

I already had a three speed fan on the aftermarket heater, but the fan was so powerful that even on the lowest setting it was cooling the heater matrix. I fitted it into the lowest fan setting, and now get a consistent warmth from the heater. It does mean I have a 5 speed fan setting!

"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10