Mixing and matching cooking surfaces works. You can stack the cooking surfaces as long as you don’t completely seal the cooking area. You probably don’t need a uniform cooking surface on a grill that big.Adrian -- that grill has a set of three grates, right?
Are all three of the grates that you have completely shot?
If not, you could get a few panels of GGs to replace just the worst part. Or get one or two grates (SS or CI) from a donor grill. Or have one part be a griddle.
Especially on a grill that wide, it would be just fine to have the cooktop broken up into a mix of griddle, GGs, SS and/or CI.
Since your grill lacks a sear burner, I think you could really use a section of griddle or GGs as a sear zone. And then have the rest be CI or SS grates for indirect use.