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.