You can continue using Plex server with Infuse as the front end. In fact I recommend it because it keeps the metadata available for Infuse. If you just point Infuse at an SMB share, the AppleTV will occasionally flush Infuse’s cache and it will have to reload all the metadata like posters. This takes time and is annoying. If you move to Jellyfin as your back end, you can point Infuse at that library too/instead with the same benefits.
You can continue using Plex server with Infuse as the front end. In fact I recommend it because it keeps the metadata available for Infuse. If you just point Infuse at an SMB share, the AppleTV will occasionally flush Infuse’s cache and it will have to reload all the metadata like posters. This takes time and is annoying. If you move to Jellyfin as your back end, you can point Infuse at that library too/instead with the same benefits.