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!

1972 4/4 4 seater, 1981 MGB GT
1984 Harley Davidson Electra Glide, 1990 Kawasaki ZX10