Do I need a stand alone streamer?

I’m hoping you’re streaming either over Wi-Fi or a direct Ethernet connection from your router rather than Bluetooth?  Anyway, it’s very possible something like a Node or Wiim will be better streamers than the one in your AVR, but whether they’d be a meaningful upgrade in the context of your system is an unknown so if I’m you I’d get one that’s returnable (Crutchfield, etc.) in case the improvements aren’t all that.  If 2-channel is important to you it’s certainly worth trying because streamers can make a difference.  Looks like you’d want to use an SPDIF digital cable and you want something at least decent because they do matter (I wouldn’t use optical because it’s limited to 24/96 resolution and involves extra conversions to optical and back).  Here’s one option that’s pretty decent and won’t break the bank (you want at least 1 meter length)…

https://sites.google.com/view/pine-tree-audio/interconnects/single-ended/nexus

If the streamer has an onboard DAC you could just use regular interconnects to your AVR and compare that to the DAC in the AVR to see which sounds better.  Hope this helps, and best of luck.

A Wiims Mini into the Toslink input on the Marantz will do

Unless I’m mistaken, you were using some form of Bluetooth to get music through that unit. So if I’m correct in that statement, then a standalone streamer, even this WIIM mini https://www.wiimhome.com/wiimmini/overview would be an improvement over what you’re listening to now. 
there are other options out there like blue sound nodes, that would probably sound better But nothing in that price range will come close to the sound quality you can get when you step up into a Innuos https://innuos.com/pulse-mini/.  
so I guess what I’m saying is get your toes wet, and then save enough nickels until you can do a dive in the deep end. No sense in wasting your time in the shallow end. 

Besides the Marantz I have two Wharfedale EVO4.4 tower speakers, two vintage B&W Matrix speakers, a Canton center channel speaker and an SVS subwoofer. 

What’s the rest of the equipment in your system?