Hi, TimC
What I can immediately think of is SWMOTECH's ALU-RACK and its extension.
https://sw-motech.com/en/products/luggage/luggage+racks/ALU-RACK/4052572004654.htm
https://sw-motech.com/en/products/luggage/luggage+racks/ALU-RACK/4052572004500.htm
This rack can coexist with OEM panniers and, there are multiple points that can be tied down, so I think it's a good fit for your use.
However, there is a weakness where the front part is bended ..., but since it's aluminum, I think a hammer can solve it 👊