Nice build!
In post 8 you show the overflows glued together. In my (humble) experience I eventually need to take mine apart for cleaning or investigation. Since this is on the "wet" side and not under pressure like the returns a "leak" is the slow draining of the overflow box when the pump is off (like individual drips).
In a similar but smaller tank I also switched over to a Herbe with an overflow and a siphon for noise and greatly increased flow.
I am not advocating taking anything apart right now, but you might want to set aside some spare parts. Murphy's law and all, if you have spare on hand you might never have the problems I experienced...