Re: using conduit between remote and local breaker box, it depends on the physical layout. If the power cable immediately exits the room from the local breaker box it probably won’t do much. But if it runs around the room through ceiling or wall for an appreciable distance it might be helpful. Cross coupling from other power feeds in the romex nest isn’t really an issue unless there is a poorly designed appliance that dumps high freq noise back to the box. And they are ultimately connected together there anyway.
Note there are likely to be code limits on how much power can be fed through the remote panel. If you have a 100 amp breaker box and there are already 100 amps worth of breakers, you might need to also upgrade the service lines into the panel to make sure it can handle the new additional load.