Looks great. I recently jumped back into the hobby after being hooked on reefer for a long time.
So I'm envious of your planning, and also what you can do with an older setup.
Just a couple observations.
1.) Might want to consider taking a piece of 3/4" plywood and cutting holes in it, and using it as a stability plate, to run those manifold outputs into. You could screw it into the side of the stand now, and it will greatly reduce the chance of cracking a bulkhead or worse, the bottom of your tank if someone gets overly ambitious turning a valve.
-I had a buddy who broke a bulkhead literally OFF the bottom of the tank, when his 4 year old threw the dogs toy into the cabinet while he was doing maintenance, and their 65lb. boxer slid into the sump.
Otherwise you could use some heavy duty strapping to staple it to the top of the stand, to add rigidity.
2.) Just another observation, but you can make your drains a little more snug and compact to the roof of the sump area, by using 2 x 45-degree unions, spliced together with a 1" section of pipe to give you more of a "sweep" than those 45's do. I have always done mine that way and it also seems to be a little more compact left-to-right. It's always a matter of opinion, but it just seemed it always allowed things to be a little more snug.
Lastly, before you button up the bottom, think about elevating your power controller / DC-8, or whatever AC power source you'll have so you can avoid splashing. Looks like you have room if you wanted, to actually mount a shelf in there now, and reserve space for wall-warts, power cords, etc.
I like using these little 1U rack-mount wire management panels, which have a removable cover, to hide all your wiring. (it's also really nice for containing 1/8" hose from dosers, etc.
1u rack mount wire manager
And on that topic, if you're going to house your ATO in that lil cabinet, you might drill a few 1/4" holes in the back panel of the stand to run in your dosing, kalk hoses, etc.
Looks awesome and fantastic job on that stand !