I may have to reassess if this has to do with something more fundamental than 11r.
I was able to solve roaming on WPA3: it can be configured completely manually as I did in each AP's /etc/config/wireless files as detailed here; or you can simply toggle two parameters option ieee80211r '1' and option ft_psk_generate_local '0' (on LuCi under Wireless roaming tab, tick 802.11r Fast Transition checkbox and untick Generate PMK locally) and OpenWrt will do the rest for you. Both methods whether you detail all parameters manually or let OpenWrt do its thing will result in roaming working smoothly. Though for Apple devices you may need to set option reassociation_deadline '20000' (LuCi: Reassociation Deadline)—that is the wisdom of the community and Apple docs, I've no fruit company products to test myself.
Unfortunately, it does not fix the reason for this thread: WireGuard connection still dies upon switching APs.