Great progress buddy! Now comes the fun part! What kind of substrate are you thinking of using? Decor?
I knew I wanted sand and wanted it to look natural. After experimenting with play sand and creek sand, I decided to stick with PFS (the larger grain, tan PFS). I swear the pool people know about us fish people and charge a premium for it in the off season. When I first invested in it years ago, I paid $6-$8/bag. This time around I paid just under $12/bag.
At the same time, I'm also thankful that I got some - talked to a couple stores who'd run low or out and didn't seem interested in ordering some more in.
Discounting the overflow portion, ~2" in a 8'x4' area would require ~5ft^3. The pool store barely had enough, 10 bags @ .5ft^3, so 500#s later, I have all the sand I should need.

I have some rocks on hand, thought about taking some from other tanks since I'm hoping to eventually downsize my tank collection, but I'm starting to think I'll still come up short. Not sure if I'll try to collect some from around my house or go buy some from a shop. I'm hoping to keep some larger sand areas open too, as I'll have phenos, fossies, etc, and would like to see them enjoy searching around in the and like they'd do naturally.
I'm also contemplating a UGJ setup, so I haven't started putting in any sand yet. It'd be easier to get the UGJ and rock work built before putting the sand in.
Currently I'm focused more on trying to get the plumbing designed. I'd like to get the plumbing functional and fill up the tank entirely, just to make sure everything's good before I start putting hundreds of pounds of sand and rock into the tank. It's still a little nerve-wracking to think about the idea of a leak.