The njp is difficult at first, but I found that doing it in an open stance (it's easier with a switched stance) over and over again builds up that rhythm. Every night, before bed, I do four consecutive (switching sides and if I mess up, I start again) 321 starters in to a njp with a combo, then I do four more with a jump-in punch to start it off because it affects the rhythm slightly, then I end with two jump-in punch starters that end with an X-ray.
By the way, I tested out your midscreen combo and it's getting 37% damage without a jump-in punch, but it's difficult compared to this one.
321, whiff dash, njp, fast force ball, 32, slow force ball, 321, slide.
Same amount of damage, but just miles easier to execute beyond the njp.
Also, here's an easy X-ray combo. It's the same as the one above until the end.
321, whiff dash, njp, fast force ball, 32, slow force ball, 3, x-ray. 45% With a jump-in punch it's 48%
You can do a 32, or 321 before the x-ray (321 is harder), but the scaling of damage actually makes it less damaging than ending with just the 3. It's easier and much more painful!
Oh and umm.... just check out some Chris G matches for some good corner combos. He has plenty.
Hope this helps!