Circulation for a 210 gal

[mark]

Nice setup and to a happy married life.

My comments from my experience and lots of reading:
-the 2" drain seems big enough and since overflow in in a separate area than viewing, noise will be a lesser concern
-can't knock big sumps, raise it off the floor a bit can ease maintenance and less head
-ref above and flowing back to display ideal. Can T off main return but a separate smaller pump from sump to ref won't take any flow fr tank
-Dart my choice on my CL, thought flow rolls off with head a bit much for a return
-have you considered your single return branching to a manifold (championed by Calfo). Multi outlets for flow and can be adjusted as tank and stock grows.
-wouldn't be concerned with Spaflex if done properly. Works for years and years in hot tubs, higher temps, massive flow, chemicals...
-pointed PHs to each other for random flow on my old tank and sure would work on a Dart and Stream in a 210g but have you considered CL (them again I'm biased).