thx ?
but, why change to imacpro1,1 ?
i think everything's working fine, even my RX590 is correctly detected with your edit - was detected as RX 580 before.
Metal: Supported, feature set macOS GPUFamily2 v1
even iStat works, except GPU temp not shown, but that's ok for me.
and
Prozessor 3,5 GHz Unknown
is only cosmetic i assume.