I would say then to go with 1" outflow to your diffuser, then split to 3/4". On each leg of these you will want to add ball valves to adjust flow, if you need to up flow a bit, throttle down on the valves accordingly. This will give you some versatility. I would have one on the 1" leg as well.
Alot of sumps also have a return to the sump for a bypass if there is "too much flow". This allows you to throttle the flow way down if necesary.
AS far as pumping.... whats the height to the tank lip from the sump?
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