Welcome to the forum. If you've had a chance to poke about a bit, you'll know there's lots of good information here. Sometimes I find it helpful to search for threads about set ups similar to mine, to see what has worked well for others.
If it were me, I'd probably want the substrate to be just a little deeper in the front. It can be hard to get some plants to stay planted if the substrate isn't deep enough. But if you like it the way it is, try it and see how it works for you.
I think that combination of bulbs should work fine. But if you don't like the way it looks, by all means try switching out the colormax bulb for another 6700 and see if you like it better. A friend used the Coralife t-5 fixture on a non-co2 20 long tank, and liked how it looked but found it necessary to raise the lights a little because of algae issues.
I think the AC50 should work fine for you, but then I've always been a fan of overfiltering.
You can always adjust the flow if you need to. You do tend to lose co2 due to surface agitation with a hang on back filter, but I wouldn't worry about that since you're not planning to inject co2 anyway. I have used diy co2 on tanks with HOB filters, though, and it's worked OK.
Diy co2 is very easy to put together, by the way, and works well on a tank of that size. You might want to give it a try. Or you could use Excel daily as a carbon source.
I find EI dosing easy to use (especially since I was already in the habit of large weekly water changes) but have sometimes tweaked it a bit. You may find that you don't need to dose the full amount recommended for a tank that size, or may want to dose less frequently.