Not that I am a great expert....
It seems as if you may have too many branches off this one main line. I would be concerned about being able to properly balance the flow...tweak one valve and you impact flow throughout the system.
I would avoid reducing the diameter of the return unless and until you are forced to do so (for example, if your return bulkhead is for a 3/4" pipe reduce right before). Reducing the diameter of the pipe reduces flow and adds additional back pressure on the pump.
My current tank (150 gal display) has two 3/4" returns. I decided to try the Maxspect DC Duo pump instead of having two return pumps or splitting one return pump into two lines. This line of DC pumps allows you to add a second impeller (included) to the pump, basically running two returns off a single motor. I am using the 9K version (
https://www.coralvue.com/maxspect-turbine-duo-9k), which is rated at 2500 gph when using both returns. In the first six months I have found it to work just fine. There are larger and smaller versions of the pump I purchased. This would allow you to have a single pump and run the returns to the display and frag tanks from one side, and feed the huge and reactors from the other.
There are others who will opine that having two return pumps can be better, in that if one pump fails you still have a second pump still running. Like you, I prefer the simplicity of a single pump. Oddly enough, for the same price as two return pumps, I can have a single return pump and a backup on the shelf at home in case of failure. Or, in the
Amazon Prime world you can get another pump delivered (in most cases) in a day if you don't have a LFS that stocks the pump you want.
Good luck!