Upgrading my streaming/digital system- what would make the most sense?

If the Grimm is beyond your budget, I would recommend the Innuos Pulse.  If you need it to be a server, too, the Innuos Zenith Mk3 would probably make you very happy.

Thank you!
I will definitely look into that.
In fact, along with the Innous I have started researching Lumin (U2/U2 mini), Auralic (Aries G2.1).  If I am not mistaken, and I might well be as I find that often the product websites are not as clear as they could be, the advantage of the Innos Zenith is that I would have it working as both a roon core/server AND as a streamer.   This would mean being able to replace my Macmini and adding a streamer rather than having to upgrade the server as well and adding a streamer. The other brands mentioned above, as far as I can tell thus far, offer streamers only, or streaming dacs, or all in one solutions (server, streamer, and dac). 
I understand that keeping all functions separate is probably the higher quality solution butt the Innous Zenith seems to be a well reviewed so I assume that the inherent compromise of having server/streamer is fairly limited (within the constraints of my budget).  In otther words it sounds like a potentially good solution vs replacing the Macmini and picking a good streamer.

This has all been very helpful to help me think about various upgrade paths so thank you everyone,  Now I am going to have to decide if to go for the dac upgrade first or upgrade what feeds the dac.

My personal experience: I found the biggest jump in sound quality was going from a Mac mini to a dedicated music server/streamer, which at the time was en entry level Aurender. My current server/streamer is an Innuos Zenith Mk 3, I'm using the Innuos Sense music app as I find it sounds better than Roon on the Zenith. 

I also have a Small Green Computer, which runs Roon for my Bluesound speakers around the house. 

The sound quality difference is noticeable between the Zenith/Sense and the Small Green Computer/Roon in my main system. 

the advantage of the Innos Zenith is that I would have it working as both a roon core/server AND as a streamer. This would mean being able to replace my Macmini and adding a streamer rather than having to upgrade the server as well and adding a streamer.

Given your specific situation this makes a helluva lotta sense, and I agree having the streamer and server in the same box is a relatively small compromise (if it even is a compromise, which I have my doubts all things considered) that comes with a big dose of convenience and simplicity. Seems like it very effectively solves all your issues in one box, and as mentioned their Sense app is highly regarded for both usability and sound quality and is continually being improved/upgraded even further, and to top it off their customer service is top notch if ever needed. Handwriting would seem to be on the wall here.

You may have just pushed me over the line!
I am very much leaning this way the more I think about it.