I personally have them be the same device, but I have a DIY NAS, so my specs are already way overkill for regular NAS duty (it’s my old desktop PC).
Assuming your home network is fast, you should be fine to split them up. I personally designed my setup to make it easy to move things around should I decide to. I use Docker containers for everything, Caddy for TLS, and HAProxy set up at the edge to route based on domain, so moving a service to another device is just:
copy relevant docker compose and Caddy config to new machine
set up network mounts for anything the containers need
point HAProxy (and my router DNS) to the new address
test
I don’t have to remember where any of the config files are since they all live next to the compose file. I also don’t need to forget which directories need to be mounted because it’s already listed in the compose file.
So as long as you make it easy for yourself to move things around, it really doesn’t matter where your actual data lives.