Options to improve digital sound

I went through three Mac minis with various diy modifications, the two most effective were mod to operating systems, shutting down unneeded services, the other was adding LPS with Uptone JS-2 and MMK. So this one route and you'd have a pretty nice streamer at this point. 

I like the idea of adding an Endpoint, Roon sound quality improved running Core on one streamer, Endpoint on something like the UltraRendu. I use the OpticalRendu finding optical conversion worthwhile. The problem with this is you won't hear the full potential of the Endpoint with your mini in present state.

So, if not into diy mods with present mini, and I don't really like your mini as I believe it has soldered RAM (RAM upgrades another worthwhile mod), I'd suggest a new streamer The SonicTransporter  I7 is a very nice streamer at that price point, remember you'll also have to invest in a 12v 4 amp linear power supply to power it. SGC has their own, the other options I'd suggest are likely over your budget. And then sometime in the future you could add something like Ultra or OpticalRendu for even further improvements. 

Adding a quality Endpoint to a stock mini is not the way to go, I know from direct experience. Your Core is as important as the Endpoint. Now a highly modded mini can be a pretty good streamer on it's own, again I know from direct experience, but this requires a good deal of diy mods. 

The OP's best route at this point is to go with a single box streamer, decide which music player to go with, Audirvana, Roon or proprietary, this will have great influence on which streamer to go with. If going with Roon, the SonicTransporter is very nice in the price range OP talking about, the I9 with optical is very, very nice if willing to pay the extra cost. And with Roon you add the second streamer sometime down the road, the OpticalRendu great match for Sonictransporters. If choosing to go with proprietary music players the Innuos and Aurender likely best way to go.

@audphile1 Roon Core is where vast majority of processing goes on, the relatively complex interface of Roon is processor intensive. CPU's working at relatively high rates creates more noise and higher latency, both are enemies of resolution  Higher latency also means greater chance for timing issues. The reason so many find Roon inferior to the proprietary music players is those music players have been designed as to require far less processing and deliver lower latency, result is both increased resolution and more analog like sound quality. 

As for empirical evidence, I've had a variety of Endpoint and Core streamers, the Cores have a far greater impact on sound quality vs the Endpoints. I'd suggest its both the processor, latency issue, and the fact Core sits upstream of Endpoint. Contrary to certain popular opinion, noise introduced anywhere in the streaming chain cannot be eliminated within the offending component by components  further down the chain, no getting back resolution, latency lost. 

I'm certainly not the first to say this, and surely won't be the last, but the streamer is right up there with the dac in order of importance in streaming chain. I don't care about all the signal and network filters, audiophile switches, lps, etc., these all just band aids if streamer and dac not optimal, the full potential of these devices will only be realized after attending to dac and streamer. 

I'd also add a totally maxed out mac mini can be a very nice streamer at a certain price point. I learned a lot from incrementally optimizing my mac minis. Optimizing operating systems, quality RAM and linear power supplies are critical for streamers, powerful processor for low latency next. The proof of need for a powerful processor with Roon was crystal clear when I was running both Roon and Audirvana with the minis, Audirvana always had superior sound quality to Roon with all three of my minis. It wasn't until I obtained my present custom build server (I9 Intel processor, Euphony operating system, Optane industrial RAM, powered by $6k JCAT Optimo ATX lps) that Roon sq surpassed Audirvana, adding the second streamer as an Endpoint/renderer added that much more. Roon is capable of sq equal to or better than very nice vinyl setups, seemingly requires more extensive optimizations than streamers with proprietary music player apps. I should add Roon sq superior to the Euphony Stylus proprietary music player app with my setup, very few Euphony Stylus users agree with this.

So, Roon not cpu intensive. Many monitor their cpu utilization in real time and report relatively high utilization with Roon vs other music players. Also I directly monitor cpu usage via Euphony OS, I can do this by direct connection to streamer or via tablet. I'm monitoring both temps and utilization in real time.

As for digital packets not impacting analog signal, laughable. Streamers don't affect latency, nonsense. 

This all a bunch of nonsense.