I think it’s more along the lines that the Radeon R-series prior to the 6800XT were just woefully underpowered.
Try this: In the game’s System > Advanced preferences window, under the API pulldown menu, select the Metal-Legacy option. This may improve performance enough for your machine to get better framerates. Right now due to the absolutely horrid state macOS is in with regard to both fracturing (users on 10.14, 10.15, macOS 11/12/13) and the updated Metal 3 specific code, performance is pretty bad. But the Metal-Legacy option was put in to help the Intel Mac users get a bit more performance at the expense of a few (still as of yet unknown) graphics effects. You may find this to be useful for you in regaining some of your performance.