Maximize Mini with Roon to match my system -- or new component?

I went through a series of three minis, unique upgrades with each. I'd say the two most substantial improvements were in order of importance, lps then OS optimization. The Uptone MMK no longer available, don't know of another solution, minus another option I wouldn't continue with mini, quality power supplies are extremely important for any streamer. If you could find a used MMK, bundle that with an Uptone JS-2 lps, extremely high quality choke based power supply. As for OS optimization I shut down virtually every conceivable service, even to extent of going into DOS and changing command lines (this can be risky if you don't know what you're doing). Hopefully you're not using wifi with the mini, remove antenna, this is noise maker. There were any number of guides as to how to optimize minis for high quality streaming, hopefully these still out there. In any case there is a fair amount of diy involved, if not willing or able, I'm sure you'll get many suggestions for off the shelf streamers.

Dedicated streamer in your price range is far better investment vs SU6 or any DDC.  DDC is marginal improvement vs a quality streamer, these icing on the cake, and then only for dac's with I2S input. I previously owned Su6 and now own Denafrips Gaia, these are some of the LAST components to purchase. At 2-3K price level the Sonic transporter would be good move. At some point later down road, separate Roon Endpoint will be further upgrade\. I've been down these roads, speak from direct experience. 

Good choice to move away from mini, the mini isn't much better than any other general purpose computer in stock form, I know directly from experience with three of them. 

Running Roon dictates your choice of streamer to some extent. I'll state Innuos and Aurrender not best choice for Roon, they're both optimized for their proprietary music player apps. At one point in my research on upgrading streamers from my last highly modded mini I settled on Antipodes as a great off the shelf streamer for running Roon, powerful processors and great power supplies,  Antipodes owners could give you advice for used models to look out for. If you want new in your price range, I l like the SonicTransporters, again we have powerful processors, add quality outboard lps and you have nice setup for Roon.

You have very limited choices for streamers with I2S output, and I'd question the quality of that I2S. In my view its only worth going I2S if the I2S clock and power supply to that clock in streamer/ddc is superior to dac's master clock. The I2S cable in itself is of little or no benefit, virtually all the benefit is found with the potential of running an external clock with superior clock and power supply to that found in dac, this being the master clock. No other input allows bypassing dac master clock.

Your speaking of full size U2, not the mini. If this case a quick perusal looks nice, haven't determined exactly what processor its using, but they claim Roon ready component, generally you'd hope this means optimized to run Roon. In any case its not a bad thing to go with Roon, great interface and sound quality when optimized, I find it's sound quality superior to my own proprietary Euphony Stylus music player app.

Not at all, your setup very nice, Pure Audio Project speakers would be on my have to listen list if I were in market for speakers. I previously owned PL Dialogue amp, the Holo is nice. You could also look into some fairly easy mods for the PL, thread running at the moment where 'gon' member modded his PL with caps and some resistors, fairly easy mod, I modded my Dialogue in somewhat like manner well worth it, especially with a nice streamer.

Per the PL upgrades the Duelund CAST coupling caps and I believe the VHAudio caps on bias board OP in that thread are A+ choices. These will provide the qualities you seek. By the way, the PL was part of my amp rotation along with Auralic Vega dac when I was playing with various Mac mini streamers, I could appreciate the difference with streaming mods with that setup.

With Roon the Core streamer is far more important than the Endpoint streamer, Core does all the heavy lifting, this has to manage the interface, indexing of sometimes huge libraries, interface with Qobuz, Tidal or some other stream provider, and it runs the actual Roon player. All this requires relatively high level of cpu, RAM compared to more basic/proprietary music player apps, so more powerful cpu, more RAM required for a quality Roon streamer. And then we have critical need of quality lps for thee streamer, this probably the most important aspect of any streamer. Running Roon core on a stock mini isn't much better than using any other general service computer, I know because I directly compared time and again, and I did many mods over many years with a number of minis. The highest possible mini I could possibly assemble didn't come close to competing with my present custom build streamer. I can decipher the design of these off the shelf streamers, the Lumin and Antipodes spoken of here will deem any mini, even fully modified, obsolete. Now, if your'e comparing $2-3K streamers to modded mini, the modded mini can compete, perhaps win here, but then you're likely $2-3K in on modded mini.