WPA3 issues

The problem with WPA2+WPA3 mixed mode is less with the AP (respectively the orbi in this case), but with clients simply not working correctly with such a network. Especially Apple- but also some Android 10 devices simply don't connect in this configuration (they can connect to a pure WPA3 network or a pure WPA2 network), fixing this will probably have to be done on the client side (in other words, it's not likely to happen). Netgear probably disabled mixed mode to ease their support burden, as it affects the most common wireless devices around.

Are there alternatives to mixed mode? Technically yes, the hardware should be able to run multiple AP interfaces (VAP) on the same radio, so one dedicated network for WPA2 and another for WPA3, with different ESSID (similar to the guest mode, just with different routing/ filtering semantics) and PSK settings. This would work, but it's obviously not the prettiest solution and quite prone to user confusion (why do I need to select two different ESSIDs and PSKs for my single network - which network do I need to connect to).