Looks great!, the idea for larger holes is good, the larger the hole the less velocity the water will have getting sucked in, meaning it sucks in the least amount of "unwanted" stuff. And also, I've never heard of people saying they have too big of holes drilled in the tank, can always adapt down, can't open them up (easily). Definitely have ball valves and unions on both sides of the motor. Rarely needs to be serviced, but it'll be night and day how things go when you have them.( I don't, it sucks).
I have the exact same setup for a sump, the only thing i would add is a better way to hold filter socks in on the skimmer side. I don't always run them, but they are kinda jammed in there when i do. The other thing is possibly adding a 4th weir to the skimmer side(i imagine will have more flow?), or keep the ATO at the level you have in the pic, which is probably a better idea
. My 1" waterfall from the skimmer weir into the return section causes lots of micro bubbles.
I like the dimensions for the display, little more shallow and wide than most, will make a great reef.