AC1200 / R6220 - 5G connects briefly, then drops and disappears from available networks; 2.4GHz fine

I am going to add my name to the list of people suffering from 5G wifi drop woes. Here are the details:

AC1200 / R6220 running latest firmware V1.1.0.106_1.0.1

Home network consisting of:

1. Lenovo Thinkpad X1 Carbon 8th gen, Windows 10 20H2 build 19042.789 *FULLY* patched, both Windows and Lenovo updates

2. Samsung Galaxy S10 running Android 10 and the January 2021 security patch level

3. Various Google devices - Google Home, Nest Thermostat, Chromecast, several GE smart lightbulbs

Problem:

When I try to connect any device to the 5G ssid, it connects briefly (15 seconds - 3 minutes tops), then drops, and the 5G ssid disappears from view on all of the devices. The 2.4GHz ssid remains available, and if connected, remains unaffected on any device. The 5G ssid does not reappear again until I reboot the router, or several hours go by. But it never reappears just by toggling airplane mode, or rebooting, etc, any of the devices. When it disappears from one, it pretty shortly thereafter disappears from all (within a minute or so).

Things I have tried:

1. Relocating my router all over my very small and open floorplan apartment. There are no obstructions or metal cages around it. Currently it is approximately 5 feet away from my laptop, on my desk, with an unobstructed path between. But I've also tried moving it across the room, etc. It is separated from my modem by several feet. It is not near any appliances, etc. 

2. I have tried downgrading the firmware, both to the previous version V1.1.0.104_1.0.1 as well as an arbitrary old version, V1.1.0.64_1.0.1 - neither fixed the problem. I am currently back at the latest version V1.1.0.106_1.0.1. I have done factory resets after each firmware change, and I always manually re-set it up, I don't import settings.

3. I even switched internet providers while this has been going on. Previously I was using a Motorola cable modem and Comcast, and now I am using a Calix 854G-1 fiber gateway from RCN on which I have disabled both the 2.4GHz and 5Ghz radios to reduce channel interference.

4. I have tried various channels, from Auto to setting specific channels based on network analysis to see what other networks around me are using. The airspace around me isn't too crowded, but still, no help. I have NOT tried fully disabling the 2.4GHz radio, though. I figure that shouldn't really matter.

5. I've tried turning off my phone completely, while seeing if I can get my laptop to connect, and vice versa, in case they are somehow interfering with each other, but no luck.

What is it with 5G wifi? It seems everyone has trouble with it dropping. I am at a loss. Again, the 2.4GHz radio remains connected and solid. I have read sooooooo many threads on here of people having the same issue with other models, and haven't seen any resolution. Anyone have any new ideas?