Just as my two cents:
I'll second the recommendation to build the substrate into a slope that is higher in the back. That's always a good idea.
I'll also second the idea of going to landscaping yards to get rocks. Just avoid any rocks with sparkly bits in them since that's probably some sort of metal that might be bad for your fish, and once you get home you should put a bit of white vinegar (or a stronger acid, preferably) on each of the rocks. Leave it there for up to 10 minutes. If it fizzes, then the rock likely has some sort of calcium or lime or some other component that's likely to seriously adjust the hardness of your water. If you're keeping cichlids then that's probably fine, but given that this is a planted tank, you'll probably want to get rid of those rocks. The vinegar test isn't foolproof, mind you, but it's a good idea. I've never actually had any problems from landscaping yard rocks, though, and they're dirt cheap. The last time I went, it was about $15-20 to get enough rocks to do rock-only scapes for 2 20-gallon tanks. Very worth your time if you can find a yard nearby.
Also, I'll add on that putting taller plants toward the back and shorter plants toward the front helps to increase the effect of sloping the substrate. The idea is to create the illusion of depth in the tank. If you don't want to move the plants later, then keep the maximum fully-grown height of the plants in mind when doing this.
As with any visual art, the Rule of Thirds can help guide your planting. It's pretty simple but it can really help make things look nice, especially when you're just starting out. This is a quick and handy guide to show you the basics: Marine Aquarium Aquascaping: The Rule of Thirds
If you're having a lot of issues with stem plants floating, you can use aquarium weights. There's some arguments about lead vs. not lead vs. metal issues, etc., but using standard aquarium weights hasn't seemed to kill any of my fish yet in 15 years in the hobby, so if they're an actual problem then it's probably mitigated by regular water changes. You can use them to hold, IMO, up to three stems together, but 1 stem per weight is probably best if you're like me and are super meticulous about plant placement. Just be sure not to damage the stems with the weights. You should be able to go to your local fish store and ask if you can have a couple weights, or they can be purchased reasonably cheaply online.
As for lights, I'll throw in the suggestion of one or two additional LED strips, such as the inexpensive Tingkam strips from Amazon. They're definitely not perfect, but in a tank as shallow as yours, their light penetration and spectrum is surprisingly sufficient for most low-light plants.
In sand substrate, a couple root tabs for the root feeders (I'm mostly looking at the crypts and the swords) might not go amiss.
There! Probably a bit more information than you were looking for, but oh well.