I'm using an Orbi RBR50 router and two RBS50 satellites in AP mode connected to my Vodafone UK router. The Vodafone router naturally has its WiFi turned off and both RBS50 satellites connect directly to the RBR50.

Previously the Orbi would randomly reboot about once a week. Since upgrading all units to 2.7.3.22 it now reboots on a daily basis. There is no logic or pattern to these reboots.

On the older software I tried disabling both daisy chain topology and fast roaming and enabling both implicit beamforming and MU-MIMO. I've kept those settings when I updated to the new firmware. It's made no difference previously to the stability of the network.

My next step is to factory reset the units and start all over again. Should that not work, is there anything else I can try before replacing them with something else?

The units were second-hand and a few years old, so aren't eligible to be RMA'ed.