The powder should do pretty great as the only substrate. Not much else is actually needed, IMO.
I got super stoked when I seen the ato sitting on top. I thought man, this guy means business! I'm setting up a reef also so that's understandable
JuanSan pretty much nailed DIY co2. It's far to inconsistent for high tech, IME especially for such a nice tank and substrate choice. I may be wrong but I thought shrimp didn't do well with Co2 injection? I don't know anything about shrimp so...
I've heard a lot of good things about Monte Carlo as a no/low co2 carpet plant. Looks like larger hc if I recall correctly. Although, with dhg, if your emersed carpet is dense enough before flooding it would probably sustain itself for a decent time especially since you're using aquasoil. Dhg without co2 grows super slow, IME, regardless of lights and ferts. It doesn't really die or melt it just kind of sits their as if in suspended animation. I now have it in my high tech 26 bowfront and I'm trimming about 3 inches weekly, in regular ADA aquasoil, 4bps co2 injection, 2 Finnex 24" 24/7 on max for 6hrs, ei dosing.
I've also heard of people "dosing" plain ol' carbonated water in smaller systems like this introduced as new water during a water change or as daily top offs but that goes more along the line of the DIY Co2 meaning it usually only works well in lower tech supplemental situations (anubias, java fern, other low co2 demanding species). Excel should work just fine for most planted tanks.
If you end up towards a lower tech tank, I think a buce carpet would look real nice coupled with some mossy driftwood and needle-leaf java fern (or other grassy bush-like plants) in the background. Maybe add some stones for depth and definition. Actually, that description wouldn't necessarily be limited to low tech.
What other plants are you planning on adding, if any? Great start with the sweet equipment!