I have the Marantz NR1510 and I use the HEOS app all the time for streaming Spotify and Tune-In radio. The interface isn't great but when I use Spotify to stream I'm using the Spotify app to control what I listen to. I only interface with the HEOS app to adjust speaker groupings and volume. I have the HEOS app set up to group the main speakers, remote kitchen speaker, and back patio speakers so when the wife and kids want to listen to music all she has to do is turn on the Marantz receiver and select HEOS. She controls everything through the Spotify app at that pont. She rarely opens HEOS app to adjust the individual speaker volumes in the group.
As for a streamer upgrade...I bought the Cambridge Audio CXN V2 and I use that as my primary streamer for Radio Paradise and some select European electronic streaming channels and as the DAC for my CD transport. I've found that Spotify sounds different if I stream through the CXN first and then input into the Marantz vs streaming directly via the Marantz. Spotify sounds a bit choked on the CXN but Radio Paradise sounds great. Vice versa when I use the Marantz.
Anyway, there are ton of streamers out there that might be an improvement over the DAC in your Marantz but you might also find some streaming services sound different on either one. I do, but my wife doesn't notice at all.