Don’t worry about the fire brick if it gets a bit red. Worry about the stainless and mild steel parts if THEY get red. That is bad.
I prefer the IR gun because you can instantly get readings on various parts of the stove. I to get readings in the 600/700 range depending on where I take a reading.used to use a stove top temp gauge but I use an IR gun for that now. A good fire will put my stove top between 650 and 750, and then cruise down slowly from there.burn tubes Have never started glowing red in a way that would worry me but I think that’s the first time I’ve seen the brick glow a little bit. It was pretty cool. The stove is done a pretty good job since I bought it 10 years ago, even after I over fired it one good time.
IR gun will work on the exterior of the stove but can't "see through" the glass.
Also, they average temps in the "cone" they are scanning. ie 8:1 ratio is looking at 1" area if you are 8" away. Many IR guns are 4:1 the farther away think shotgun blast size of area that it's looking at.