Unfortunately, DSR with FFXIV (at least for me) is kinda a bust as it misaligns the mouse cursor to the buttons/clickables. The higher the resolution, the alignment gets worse. Certainly made the game look a lot better (4k was real nice on a 42") but it put me about 1/2 inch off to the right to where I needed to click the button to close windows and such. Never mind the tax it has on the graphics card itself. Plus, like the OP, the UI scales to the 4K and at max %, it's not big enough to read text reasonably and somewhat blurry (I feel at least).
If they could get the UI so that it can have it's own separate resolution, it's possible that might work out.