“the Pro Stick using either a long USB run, or also colocating the RPi and using Power over Ethernet.” is, IMO the way to go.
Some considerations might be:
Take the existing FA Pro board out of the case, apply aluminum heatsinks to the critical parts, then seal the boards using either epoxy, elastomer, or other waterproofing material, leaving only the heatsinks exposed. The USB connector can be made moisture resistant by using a boot of some rubber or appropriate elastic material, or simply applying electrician dielectric grease to the connection.
An RPi, Orange Pi, or similar can be treated the same way, with heatsinks on the critical components and then seal the board with above described method and unused ports with rubber protectors.
48v POE to a 5v/Ethernet converter is desirable as the converter can take a wide range of voltage from 10v to 48v and make it 5v. That means voltage drop over relatively long CAT5 runs becomes insignificant to the resulting voltage and current feed for the computer and stick. Wall wart style POE injectors are $12, and the voltage converter/splitter is $12.
The FA Filter is quite effective, and can be integrated into the stack forthwith.
The antenna can be added to the stack and be a J-pole, coaxial dipole, collinear, or coaxial collinear, all of which will fit in a piece of PVC pipe along with the electronics at the bottom, and a weatherproof RJ45 jack at the bottom. Add some appropriately angled venting to the sides with a drip hole at the bottom, and this should withstand most climates.
Outdoor rated/direct burial/UV safe/waterproof CAT5 is about $0.07/ft. in bulk or $25 for 100’ with connectors. ($0.25/ft.)
Off the shelf, this can be built for around $100. In mass production, a combined package could be much lower. No coax, no jumpers, lots of planes.
If you’ll send me the electronics pieces, I’ll be happy to put it all together for a demo