Flux seems to work well with Ultimate SD Upscale, but I was struggling a little to make sense of the workflows people were posting. Hopefully, the workflow you can find here is more informative and easier to understand so that you can tweak it to your preferences.
Time to move on from 720p:
Which to be fair looks pretty good..
And say hello to 2880p:
Which looks even better! (Most of the time)