Not being much help, but what of a Carbon monoxide sensor? As it's in the garage. And as for the humidity is an all ways problematic issue with all electronics that are needed to not be sealed to detect smoke.
As the sensor chambers need to be exposed to detect smoke particles passing an ionization chamber of sorts, and CO2 detecting the air. CO2 can detect a fire by the offset in the atmosphere as well should be more acceptable as being mounted in the garage.
I had typed thoughts on the ideals, tho it was more towards rambling than providing useful info, mainly about issues with temp sensors needing to be in areas that would be effective/ time responsive, and PIR sees fire as flickering value on output. Tho I've never used a PIR.
Am aware that it can "see" heat as a thermal to value, then a controller works with that as per programming.
The PIR pointed towards key areas, such as a connected garage, towards the entry to the home per example to give warning for that area. Vehicle engine temperature when parked in garage may also cause some problems with an PIR seeing heat rise.
Tint on the windows would help as mentioned to prevent some ranges of false triggering.
And of coarse, I think you are clearly aware that a controller can trigger an out response to another system (household alarm) either a direct connect or an relay, Sealed Reed types often have good time on and can stand some heat in an room before quitting operation.