For the double wall pipe a condar probe thermometer is the only option and it is a good meter. The condar is made for measuring flue temps inside a double wall 6" pipe it is relatively accurate and very easy to install. I have one in my double wall pipe too. You simply drill a hole (of specified size) through both pipes at a certain height above the stove and then chuck up another specified drill bit and use the first hole as a pilot to enlarge the hole in the outer pipe only. The probe meter just slides right in and sticks with a magnet. Works great and is the only really useful add on meter to a BK stove setup. This chimney doesn't know what stove is beneath it, you are monitoring the chimney for safe high temperatures as well as safe low temperatures to be sure you are above the creo forming range and yes, the BK will make some creo.
For stove top meters, I tend to agree with webby. They are of limited value even without fans running. This is mostly due to the way a cat stove burns the smoke from a cold fire at the catalyst to make most of the heat. I have a stove top meter on mine, many brands are available for low cost and they just sit there.
As mentioned, you already have a cat meter. It is only important to know whether it is active or inactive.
Get used to setting your thermostat based on desired output and loading your stove to the gills. It may not seem intuitive but you will stuff that stove full and then dial heat output to where you want it with the thermostat. That's all you do to operate this stove, kinda boring right?
I'd be a little scared to drill holes into the chimney pipe. How do you seal it up to make sure there's no leakage around the probe?