I think the plumbing plan is pretty good, but the are a few adjustments I might suggest
1 - the main drain line could use a small angle here.
We have a very similar setup on our system, in that we are using the same overflow box and have a horizontal run on the drain pipes.
One of the issue we ran into was air getting trapped int the line several hindering the pipes ability to drain water. To fix that, we put a slight angle on that horizontal run, so as to allow air to want to travel back up toward the drain. doesn't have to be much, but just a slight angle, maybe 3 to 5 degrees
2 - Secure the pipes
Especially the secondary and emergency drains, sine they are not actually attached to anything. the main drain is likly OK, since it connected and anchored at top and bottom, but securing it too, wouldn't hurt, and could onyl help.
3 - OK, so a shameless pitch
- Whats you plan for the return nozzle?
Looks likee you have a SICCE pump in there (good choice) and judging by the size it is likly a 5.0 ? If it is, and base don the fa you have a single inlet coming in for the right, I would recommend 3/4in Random Flow Generator® pointed diaganly across the tank.
At the 800-1000GPH that pump will be able to deliver to the outlet, combined with a 3/4in RFG, you won't even need a power head
We have a kit that includes everything you need (RFG, Loc-Line and Bulkhead fitting) and works with both slip and threaded bulkheads - although it looks best on a slip bulkhead
vividcreativeaquatics.com
If its a smaller pump (say a sicce 3.0) then go with a
single 1/2in RFG