I'm about to add a new mesh network to my home. I currently have an Asus router connected to a cable modem that provides wifi. It runs with a DCHP server serving IP addresses 192.168.1 - 250. I have both ethernet and wifi devices connected.

I want to add the mesh network, giving it a new subnet and keeping my router running, while still serving up the same IP (subnet) on the old router. The new router, I'd like to use a new subnet and then slowly migrate off my current router to the new one.

The new mesh devices (Deco M5 - TP-Link) suggest direct connection to the Modem. So, I'm hoping to connect the new deco to the modem, a switch to the deco, then plug the old router into the switch. Assign a new WAN port IP (or let the Deco's dchp server assign one). At this point i'm hoping all my devices would continue to function as is. The exception I'm thinking would be any port forwarding. (any suggestions on how to port forward over multiple subnets?)
Would this work as I defined or if not, what is the best approach to allow all my currently devices to continue to work as is, while slowly migrating over to the new Mesh network?

Thanks!