It sounds like you’re trying to limit the maximum heat in your
working piece, and as John explained you might not want your torch to
be cooler (since you still want to do the hot work as quickly as
possible, minimizing oxidation etc).
So you could try to arrange a non-contact thermometer, the IR-laser
types tend to be built with two different focus lengths and you might
be able to get pretty quick feedback although most of them take
1-2seconds to register a temperature and you’d probably have to keep
the work piece clamped with the thermo also clamped and aimed
accordingly, but that approach seems error-prone. Probably better to
accept a couple failures while paying really close attention. Perhaps
some old-timers have some analog tricks such as a burn-away material
next to the piece. The problem seems similar to forging aluminum in a
coal or gas forge.
If you’re trying to speed up the work, you might pre-heat the work
pieces on a warming plate (non-critical). Anything that normalizes
your process (ie gives you the same amount of time under torch no
matter whether it is summer or winter), might help"you can count to
the same number eachtime once you put your torch on the piece. And of
course a smaller burner/tip might help save some fuel along the way.