@Flyball 

Hello. If the M5s behave like the P9s, in AP mode, the wireless devices will see the wired ones connected to the router

Many thanks for the replies - looks like this might be a suitable solution for me. 

@Flyball one thing you should be aware of, if you want to use the guest WiFi network feature of the M5, devices connected to the guest network will also be able to see the devices wired to the main router.

My topology is similar to yours, so I worked around this by plugging in a spare router to the M5 secondary ethernet port, and the wired devices into that secondary router.  The wired devices are now hidden from the guest network.