I don't have any experience with pressure filters to know if they would provide enough flow to push water through a box filter and onto your falls, and likewise, whether a box filter before the pressure filter would be advisable. Sorry I can't help you there.
And not knowing your current waterfall set-up, to know if you can or want to rebuild it... A filter falls , similar to this one, could be a suitable option for you:
The type shown above is designed to attach the liner under the spillway, but there are others on the market which have a longer, solid spillway to direct water over your falls without liner attachment. Depending on what your needs are.
The above unit is similar to what I use, and is my only means of filtration for my 2000 gallon pond. Mine has a water inlet at the bottom. It has a built in swirl chamber at the bottom with an insert across the top. I placed a thick matala type pad in atop that, with a large bag of bio balls stacked on top of that. I never rinse this pad or the bio media so that those surfaces are available to grow and harbor good bacteria. Then on top of that I place some combination of other, finer, filter pads to remove sediments. These pads are the ones I rinse and reuse as needed. Again, you would have to study further to know whether your current filter outflow is suitable for this type of filter falls; as it is merely my opinion that it probably would be. But at least it is an option to look at.
Good luck.
