Hi Jake, I am pretty new with Discus, have only had mine for three weeks (got small ones) but they are growing very fast. I credit the growth to seemingly ridiculous amounts of water exchange and unbelievable amounts of feed consumption. I'll share what I know.
Growing little ones needs heaps of feed and incredible amounts of fresh, warm water. Gravel beds harbour a universe of bugs, discus don't like them. Also, like any filter gravel beds need cleaning which is labour intensive (not for me) Bare bottom tanks can look good (IMO) you can still have plants, just don't paint the underside of the tank blue (like I did), I suggest black. Use google for ideas.
For your sump, look at the first section for good, easy cleaning mechanical filtration, followed by lots of biological filtration. I can recommend MBBR filters with K1 media. They do not build up filth like other biofilters and its worth checking out.
You can have a look at my journal if you want, but It's work in progress and is pretty far from my idea of magnificence Also - Im pretty fresh with discus, so take my advice with grain of salt, except the MBBR filter, that stuff is gold.