I have same set up.(only difference is mine is the stretched 585...now 6 meters)
Got the 180 ltr stainless tank... always near full, twin batts, big console, 2" A frame, but
I don't have a big issue 4 up. RC is a well built, heavier boat than many.
Have you checked for water in the hull? Undo the bung in the transom well and see what comes out when on your trailer with bow well up. Never found any in mine though. Suzi 140 only weighs 186 kg. Tabs will doubtless help below 4,000 rpm.
I'm running with 22" stainless suzi prop that has been balanced and 'tweaked' by Prop Solutions. (money well spent!)