Which stock app? Infuse is like Plex but way better. Its for my own content I have on hard drives connected to my server.Some great points you made here.
One ques, and forgive my lack of knowledge on these areas, but why would you use Infuse over the stock app? Also, how old is your Marantz? I have a 20 y/o Denon, which is a tank, works perfect but does not have HDMI, only optical. Does your Marantz have HDMI, if not, how do you get the digital sound?
My two Marantz are from 2018 and 2019. I used to have a pioneer elite back in 2008 but that had hdmi also.No dual zone like my new ones do. I wanted to upgrade to atmos for both rooms so I got rid of the old one. It wouldn't fit in my new tv stand anyway it was a huge receiver.
I used to use optical for things. I also had an hdmi switcher for years that worked pretty well. I like keeping all my components in the main room and running an hdmi to my bedroom so I can use all the same things in there.
Maybe get an hdmi switcher and use that? Might be time to upgrade the receiver if you wanted some new things. The apps for them is nice. Its nice having wifi built into them now and smart controls.