Success!
Installing the luci-proto-modemmanager package and defining a config via LuCi resulted in the internal LTE modem operating as expected. The '3G/4G' LED flashes regularly, as it did when the MF286D operated with ZTE 'stock' firmware and data flows through the LTE/WWAN interface. I did not uninstall luci-proto-qmi.
ModemManager creates entries in /etc/config/network, from LuCi entries as below:
config interface 'WWAN'
option proto 'modemmanager'
option device '/sys/devices/platform/soc/8af8800.usb3/8a00000.dwc3/xhci-hcd.0.auto/usb2/2-1'
option apn 'mob.asn.net'
option auth 'none'
option iptype 'ipv4'
option signalrate '5'
config device
option name 'wwan0'
option ipv6 '0'
Stopping the WAN interface in LuCi resulted in data passing over the WWAN interface, and vice versa.
I shall check out mwan3, if that works as intended, I will reinstall with 23.05.0 (aim is to have a set of routers 'baselined' on 23.05.0)