I have paid support as well, but at some point development will slow...
Regarding SSIDs, yes it was about limiting the number. I found that recomendation during some searches related to performance, same thing with the client limits. I had one I could turn off. Those suggestions I suspect are aimed at limitng processor loads on the APs, so older models are probably more affected. So far I wouldn't say I saw a specific improvement from that. But as you have operational requirements you do waht you need.
I had set small as well for the same reason, but I also read that the way that works may inhibit some clients from connecting if they report low signal strength. The lag time to connect the phones mad me wonder. So I set medium as a compromise. Maybe it helped but can't say for sure yet. Large definitely is not the option.
I see similar connections issues on my iphone (XR) more lately so I suspect changes to Apple's software may be part of the issue. I found Wifi imporved at home, but at work, roaming seems more problematic.
Are you using any softtware other then the web gui to monitor? I'm also using HP's IMC to manage network devices and autthentication, and it has a WLAN option that has some useful features for working with the MSM devices. You can easily see client distribution on AP maps and do some basic signal sterngth maps as well