Do the Mojave or Catalina patchers support AMD PC GPUs or do you need a flashed card for this?
'older Mac Pro'… specifics would be good.
However, the R9 270X in its HD 7950 guise works fine on a 4,1 with firmware upgrade &amp; a standard 5,1. With an unflashed card you must not use anything which needs the bootscreen - filevault or EFI/firmware lock of any sort.
The smart money was always on keeping the original video card for situations such as these, though neither the GT120 nor the HD5700 can actually run Mojave.
Anecdotally, Mojave is better than High Sierra imho, but personally I wouldn't push any cheesegrater to Catalina.
You'd be wise in any case to very carefully read dosdude's page on Mojave, as it contains a lot of caveats dependent on the precise Mac Pro model.