Lots of good suggestions, and I’ve tried many. Stay away from Bluesound and HiFi Rose. The Bluesound sound quality (yes, even the Node X) is pretty bad. HiFi Roses sounds good, but the app is a nightmare.
Eversolo DMP-A6 and DMP-A8 are really fantastic overall, and then exceptional when price is considered.
Agree with many others that if you are keeping the Marantz, the Wiim Pro would be a great choice to just use as a transport.
I haven’t had the Sonos Port, but the Sonos software is really good
Best of luck!