Couple unrelated topics.
Pi 4 temperatures - 110°F is about 43°C and is a very comfortable temperature for a processor. I have a Flirc aluminum case for my 8GB Pi 4B and have stress tested it and it keeps the CPU/GPU within acceptable limits when fully loaded. If you add active cooling, how will you monitor it so that when it fails, your system will not oveheat? (It will throttle before it gets too hot.) I suspect that PiHole is more likely to be I/O bound than CPU bound and is less likely to stress the CPU.
Catadioptric lenses - I have a 500mm cat that came with 2 or 3 neutral density filters that screw in to the inside portion of the lens. They’re about 1" diameter and would be less convenient to use than an external ND filter. These would provide some exposure control. Depending on cameras, you can also minimize ISO setting but none of my Nikons went below ISO 100. Thanks for mentioning these. I should give mine a try on my D7200. It would not work easily work on my D50 or D80 but I think the newer one is more flexible in that regard.