I added an orbi mesh system in the house at the beginning of COVID and have been really pleased. Now I'd like to add ExpressVPN to the network. From what I've gathered I can't install it directly on the Orbi system and need to install a router (or another device) that supports tomato or DD-WRT upstream of it. Any idea what my best option is for doing this? I'd probably like something that is at least 500 mbps. Thanks.

I believe any of them should work because you are going to turn the WiFi in it off and use the Orbi's as WiFi AP's and not a router. It is just going to be wired to the Orbi. Good luck with the setup. I have NordVPN with a dedicated IP and am using a NetDumaOS based router with a similar setup. YouTube and Amazon can still detect that I'm on a VPN even though I pay extra for the dedicated IP to stop that. I'm dealing with customer service now. We will see...