Those 2 90's and the pipe running horizontal as opposed to runnimng down hill, put back pressure on the drain. This slows the flow dramatically. Because its a drain and has no pressure other than gravity. Increasing the pipe size and allowing the water to flow without the restriction of the 90's down hill, will allow the drain to handle more water. You could do the mag 9, put a BV on the return line and throttle it back if need be, or divert some back through the fuge if you so desire. While you're likely not going to be able to handle all that it will deliver, you should be able to most of it.
It looks though that you have a pretty striaght shot from the sump on the return, but your still not getting 600gph through the DT to sump, you should be able to process that much flow through the DT if you upsize the drain pipes etc. You could just use say 1 1/4 spa flex, (or the same stuff you currently are using in 1 1/4) and connect it as straight to the sump from the bulkhead as possible.
BTW, I use a mag 7 on my 65, with a HOB "u" tube OF box. My OF will just barely handle the pump wide open.