I like that backflush idea
Would be simple and easy, but I don`t think that would work out well, at least it never worked for me.
Problem is, the flushing water will take the path of least resistance and once found, will leave the bulk of the pleats still dirty.
Looking through the postings, I agree with getting your sumps mechanical media more involved with debris removal.
If the media your using isn`t trapping out a bunch of the gunk, that leaves your cartridge system doing all the work.
One of the beauties of a sump is the ease of media changing.
Pretty sure you can find a combination of materials that would take some of the load off of the cartridge.
Heck, anyone who can put what you have together can find a way.
Good luck.