One question, since you are to add another return pump, where is the other return pipe gonna be?
<4000L/h should be Ok which in your case includes headloss and output from FRs.
I can only give a rough gauge from my experience with my tank but from the pictures your sump seem pretty small and water level do rise significantly, I would encourage you adding ball valves as when tuned down, more flow will be directed to your other output like FR
Lastly, headloss calculator does not calculate you T off so you might want to include additional horizontal length and pipe output.
Good luck