@Joseph-TP 

It's a little hard to explain but I hope this is understandable:

1st floor:
ISP 1 modem --> ER605 (but load bearing is no longer used) --> 10Gb switch
ISP 2 modem --> AX1500 (no wireless turned on) --> 10GB switch
Both ISP 1 and 2 modems connect to a 10Gb TL-SX1008, which connects to my 2nd and 3rd floors, and to my first BE19000 in the ground floor living room with wireless turned on.

SSID is one and the same for 2.4 ghz and 5 ghz, 20/40 MHz, 2.4 ghz channel 6 / 5 ghz channel is 40, 20/40/80/160 MHz / 6 ghz channel on Auto, PSC enabled, up to 320 MHz

IoT SSID is different, and only runs on 2.4 ghz.

MLO turned off (I experimented before and got slower results compared to the non-MLO unit on an iPhone 16 Pro Max). Both RX and TX flow control for both Internet and LAN menus are turned off. 

2nd floor:
ISP 3 modem --> second BE19000, wireless turned on. same settings as the first BE19000 except 2.4 ghz runs on channel 1, 5 ghz channel is 36. All devices and computers meet together via various TP-Link and Trendnet unmanaged 8-port 10GB switches.

3rd floor:
Wall plate keystone --> third BE19000, wireless turned on, same settings as the first BE19000 except 2.4 ghz runs on channel 11, 5 ghz channel is 44.

Everything is wired with Cat6a or Cat8 cable basically. There's a BE19000 on each floor. I could never figure out what the combination of things would cause this, and it happened months ago. Whenever it would happen, all devices would suddenly have no more connection to Wifi. iPhones and iPads would show cellular connection. Tapo cameras would turn red. Searching for the SSIDs on iOS, they were all gone. Once it happened, it could affect one BE19000, or it could (less frequently) affect two. I could not verify a lot of things well enough though as I was anxious enough then to get the network up and running right away. Rebooting would result in the SSIDs showing up again.

I tried fiddling with settings using trial and error, and noting what the last stable configuration had that was different from the last configuration that had a problem. After this happened around 4 times or so over a period of 2 weeks, the behavior finally stopped when I turned MU-MIMO off on all units. The network then became stable for a few months, even without rebooting.

Wired connections still worked when the SSIDs disappeared. All BE19000s were running in router mode as I wanted consistent and predictable browser access to each router's GUI via their IP addreses.

I've since talked myself into updating all of the routers. If the SSIDs disappear, I'll follow your suggestions. 

  @Joseph-TP 

More than 24 hours later, all wireless SSIDs in my third floor unit are gone. All wireless devices are reporting no connectivity, even those on the standalone 2.4 ghz IoT network. 

2nd and 1st floors are okay. But I think whatever is causing this issue, has yet to be resolved. All 2.4 ghz channels are now running on 20 mhz. 5 ghz channels are spaced appropriately (channels 36, 52 DFS, 64 DFS).

I will have to turn of MU-MIMO for now to hopefully return to long-term stability.

  @Iainyeo 

Is MU-MIMO enabled in your case?

  @Corpski  Yes MU (MIMO) is enabled.

  @Iainyeo 

Ok I don't know if you're up for this, but try to set your units to OFDMA only (no MU-MIMO). Feel free to share if you still experience SSIDs disappearing.

  @Corpski 

MU-MIMO is the techology that makes wifi7 faster. I am abit reluctant to disable that to be honest. Today seems fine for me but if it happens again, I will give it a try to disable that. Will update you again.

  @Iainyeo 

Of course. In my opinion, it's true that MU-MIMO is a good enabler of Wifi 7 performance, but it's much more felt for really busy networks (like 4 people streaming or requiring low latency gaming at the same time, etc.). Other primary driving factors are 320 MHz channels, 4096-QAM, enhanced OFMDA, and MLO (when it will be optimized). Since most of my devices are wired, my network is nowhere at a point of intense saturation, and OpenSpeedTest runs show that  mobile and laptop devices are still running at their maximum expected speed for now. 

If you have a multigig or 10GB network, you may be interested in testing your speeds with and without MU-MIMO turned on, and in multiple locations and pairs of devices at the same time to see how fast your network is able to handle the load. It might be a workable, stable option for the meantime.