[ChemGuyEthan]

I'd consider dropping the gourami and definitely the angel, at least. The tank is frankly too small for an angel. As they get bigger, and longer fins they'll want ample swimming space and in my opinion, 21 gallons just won't do it, especially with all the other fish. In my early fish-keeping I had an angel in a 20 gallon column tank and he turned super aggressive once he was fully grown I think mostly because he was crowded. Not to mention they can be aggressive in general, tight spaces or not. So to me, the angel is a no go. The gourami is hit or miss, the problem there may also be aggression either on the side of the betta to the gourami or vice versa. Both fish are mid-upper level dwellers so they could be in each other's way. I've also heard stories of barbs nibbling on betta fins. Since bettas are very slow swimmers, they're easy pickings for fast, hungry scavenger fish like tiger barbs. Not sure if the danios are the same. Tetra are good options as they're very docile and come in all sorts of colors (rummy nose, penguin, neon, cardinal, etc). I think consider one, slightly larger school of fish (maybe 6-10 depending on size) instead of two small schools. The final bit of advice I would give you is, assuming the betta is the most aggressive fish with the others being shoaling fish, add him last. If he's in the tank first, he'll think the whole territory is his and then when you add new fish, he'll chase and attack them cause they're invaders. But if they're in it first for a few days to settle in, once the betta is added he'll know that the space is shared and likely not be aggressive towards the first inhabitants. Shrimp would be excluded from this rule, though keep in mind the betta may eat a few baby shrimp here and there, which isn't really an issue with prolific breeders like cherries. Anyway, hope all that is useful.

As far as plants, that sounds good. You may need to dose Fe as well.

Welcome and best of luck with this setup!