Very nice work on the stand. I might suggest, as an added precaution, a vertical center brace. The tank will weigh in excess of 300 lbs. A little overkill on support could not hurt.
Get a riccia fluitans (crystalwort) culture going. A bright light will and a little CO2 will get a floating stock of this to grow explosively. Once you have enough either tie or use the SeaChem tissue culter glue to adhere the riccia to the slate and then start 'carpeting' your foreground. Eventually you will have a lawn on the bottom of your tank that you fish will seriously enjoy... I did this and they played in it, bred in it and their babies hid and grew up in it. It's nice touch and your tank will benefit from the O2/CO2 exchange of a large area of live plants.
<and now I have seen your pics>
So I see now that you have added growth media and great spread of plants! Really nice!
That said, you might still try the Riccia trick to add to your ground cover.