Worth flagging a catch with the per-app rule approach: blocking the app on Wi-Fi isn't the same as forcing it onto 5G.

Android routes an app over whatever the default network is, and that's Wi-Fi whenever Wi-Fi is up. A per-app firewall rule denies the app on Wi-Fi — it doesn't reroute it to cellular. So the likely result is the app has no connectivity at all while you're on Wi-Fi, rather than quietly using 5G. Only the app itself can pin to a transport, by requesting a network with TRANSPORT_CELLULAR, and almost none do.

For the lag you're actually describing — commands dropped during the 5G/Wi-Fi handover — the thing that usually helps is stopping the handover happening at the moment you press the button, rather than per-app routing. Worth checking whether your smart home app has a local/LAN mode that talks to the hub directly instead of round-tripping through its cloud, since that removes the problem instead of working around it.

Happy to be wrong if someone's got Rethink actually moving an app onto cellular while Wi-Fi is connected — that'd be useful to know.