The only consistent part of my setup that I hadn't tried changing was the F to TS-9 adapter. I ordered 2 new ones that arrived last night. After trying many configurations, it looks like the original adapters were broken. Regardless of RG6 length (5', 50', 75') or number of antennas attached (internal, 1 or 2 external) I was able to get consistent download and upload speeds.
The only thing I'm concerned about now is that the TS-9 plug is extremely fragile and doesn't seem to have a retaining feature. As a result, the plug slides on and, since the wire is fairly heavy compared to the hotspot, things feel like they will be pulled off the table. When I get the final mounting location, I'll definitely have to retain the RG6 carefully so it doesn't pull on the connection less it will unplug itself.
One interesting thing I found was related to speeds. When I use my phone (Samsung Galaxy A10e) I get speeds in the range of 35-55Mbps download when connected with either 2.4 or 5GHz (so I assume 802.11n or 802.11a). But when I use my laptop (an old Dell with 802.11n) I consistently see speeds in the range of 22Mbps maxing out at <30. Those seem more consistent with 802.11g real world speeds I've seen quoted. I need a new laptop anyway, but I'm curious if there's some reason a device that can run at n speeds would throttle down to g unless the protocol says "only work with the speed of the slowest device on the network". I know the old routers (first generation g) had settings to allow b devices to work but the network would run slower.