Do I go with:
1.25x DSR with __%? smoothness & MSAA 8x
1.5x DSR with __%? smoothness & MSAA 4x?
2k DSR with __%? smoothness & MSAA 2x?
3k DSR with __%? smoothness & no AA?
I would assume FXAA would be dumb because the whole point of DSR is getting an ultra-sharp image and taking a blurry image and downsampling it seems like it would just produce a blurry image.
DSR itself has blur queued into its final effect on screen, using it along side FXAA is indeed counter intuitive. If you want to go with DSR, I'd suggest 1.78x DSR (and smoothness at 15%, but do check your preference) with MSAA 4x.
But I'd myself just go with GeDoSaTo (when its possible) 2560x1440 -> 1080p, with Lanczos and FXAA (in-game). Because this combo would be faster than DSR + MSAAx's any time and GeDoSaTo_2560x1440_Lanczos_FXAA overhead isn't high.
Edit: One problem with DSR's smoothness factor at non 4xDSR settings, is the fonts in the game..they'll look all weird.