Teetosugars, I’d be interested to hear how rigid your rack is with those long stays reaching down to the seat stays. I anticipate the same problem with my Fargo which has an even more extreme top tub slope.
My Roberts has a horizontal top tube and the stays are pretty short, it makes for a really rigid platform. I don’t know why frame designers insist on sloping the top tube knowing that rack stiffness will be compromised. On my old Dave Yates I fitted a Blackburn expedition rack, you could virtually get the rack tight against the rear brake cable and the stays were super short. No flex at all.