Unless I misread (and I read this over and over), how does 1 iPhone stop all devices...and how is this related to OpenWrt?
From reading this, I'm getting your partner's iPhone is jamming all devices - preventing them from connecting to the AP; and somehow you determined that it wants a mixed AP with PMF. Is that correct, or was that a miswording?
I would advise:
require PMF if you have not done so (it's defaulted to optional in OpenWrt version 21)
a WAP3 only SSID
a WAP2 only SSID
connected to the same LAN.
I'm not sure you've realized the issue:
It seems some of your WAP2 devices don't support 802.11w required management frames - don't blame your partner, you'll have a longer relationship and less frustration with your router.
You could set up a second AP interface (different ESSID) on your radios, also bridged to lan. One exclusively for WPA3SAE/ PMF=mandatory, one exclusively for WPA2PSK. WPA2/ WPA3 mixed mode is known to confuse quite a few (broken) clients.
What I'm saying is that none of my wpa2 devices (as in the ones that don't support beyond wpa2) will connect to the access point with wpa2/wpa3 mixed mode and PMF set to required.
They connect fine with PMF set to optional. But then the iPhone won't connect.
I don't think the iPhone is jamming anything. Just that it's an annoyingly arrogant product that thinks it knows best ;-O