I have same problem like you. 70 odd permanent IoT devices tenggelam timbul at their own convenience, not at my convenience. Maxis' Huawei LG8245X-10 mesh.
Difficult to manage since we're not staying at the property every day.

I found that if i have only 1 SSID on the mesh, and all IoT connect to that, they will come and go. I find that about 20 devices per SSID seems to be working to make them stay online, thus I had 4 SSID on the same mesh.

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I switched to TP-Link Omada.
I bought ER706W gateway and EAP613.
I was surprised that when I initially installed just 1 EAP613, it immediately took over 4 IoT-related SSID and 1 CCTV SSID without breaking a sweat. All devices connected and online, 24/7.
I have also moved all the IoT devices back to only 1 SSID.

It made the 4 previously-configured Maxis'  a bad preschool joke!

I resolved to expand further on Omada. Replaced the root LG8245X-10 with ER706W. Bought a 24-port PoE managed switch and moved all my wired CCTV and devices to it as well. Now my network is split into VLANs for Security, Automation, and for interactive users. All managed from central and accesible from web and app.

The only beef is, it takes 5-10 minutes for network to be usable if reboot the Omada gateway. But other than that, no complain.

Caution:

Access Points of Omada are wired-only backbone.
But they are VLAN-capable.
Each AP have limit to the number of SSID they can deploy; and each SSID can have their own VLAN, thus separating traffic and priority right at the AP.
EAP613 can deploy maximum 8x SSID on 2.4GHz and another 8x SSID on 5GHz; that means you can have between 1 VLAN to 16 VLANs delivered on wireless.
Depending on model, they come with choice of PoE or power adapter; no difference to deploying wired CCTV.

it's not so much about the 1 SSID vs many SSID.
it just so happened that when i was using Maxis' mesh LG8245X6-10, which is capable of creating multiple SSID, the issue almost disappeared - ALMOST - when i divide into 20-odd per SSID on same mesh network.

i would attribute the problem to these:-
- consumer wifi devices are only capable of handling limited number of concurrent clients. processors just fast enough to turn around a limited number of clients, not to mention having to fight for limited aurtime slot against other APs in hostile vicinity.

- IoT devices are still on the 802.11bg protocol, so they can't benefit from newer, efficient time-sharing protocol.

- IoT don't transmit huge packet, and most of the time idling and sleeping to conserve batteries and what not; probably can't wake up soon enough for their allted transmission slot.

- the flapping issue becomes prevalent if a network has been left running too long, that eventually even my static IP reserved for CCTVs and anything will become not responding. this is educated guess:- when IoT devices reconnect, they'll request new dhcp address, not reuse what was previously already assigned, and my dhcp server kindly and foolishly dish a new address. so to make sure there is always an address available, i shortened my dhcp lease time to 2hrs, which helped to further mitigate the issue.