So, I believe in the separation of concerns argument and I prefer to have my streamers separate from my core, and leave my core in my network cabinet. Also just helps with phsyical setup in my case, but many have said they thought that sound quality benefited too. I don’t own an HDMI DAC, and I like my DACs a lot, so there’s that.
Simplest bridge for you is an RPi loaded with Ropieee. There’s lots of threads here, the project owner is all over these boards (there’s even a special topic just for Ropieee. There’s other RoonBridge installs (DietPi, Roomio, etc) but I prefer Ropieee, ymmd. There are lots of install help guides on the forum, the granddaddy of them all (which isn’t entirely up to date) is here.
Without a HAT on the RPi, you can go USB-out into a DAC that supports that. That’s the simplest, and what I do for most of my machines. If you prefer, you can get an Allo product either with a DAC or which outputs RCA/TOSLINK, or even a Pi2AES which outputs basically everything (AES, TOSLINK, BNC, etc). That’s what I have in my main stereo. All my endpoints are either Sonos (simplicity) or Ropieee (also simplicity, but my family doesn’t like Roon).
Simplest thing in the world is to pick up a decent off-board DAC (examples: Schiit Bifrost, iFi HipDAC, SMSL M500 if you love MQA), a flirc and RPi, and feed the DAC with the USB from a Flirc-encased RPi running Ropieee. Then you have a semi-portable headphone setup, or could feed the Marantz. Lots of audio tomfoolery to experiment with here, but keeping separate devices allows you to play around with more as you learn what you like.
Good luck!