MacPro mid 2012 5.1 - I bought Radeon Gigabyte 7950 3GB 4K Graphics Card (Mojave 10.14 compatible), but it seems to be incompatible with AMD ProRender for Blender. Any suggestions for a GPU please?
the card you have might work. but might struggle a bit with only 3gb.
as far as a good card to recommend it really depends on your budget. AMD RX 580 is not a bad card to go with. I'm not sure though if its compatible with Mac OS though
I'm can confirm the AMD RX 580 8GB is a great card on macOS as it was the one Apple chose to develop and test eGPU support on macOS. It runs RPR really well and is very optimised.
A VEGA class GPU is obviously quicker again but not anywhere near as well optimised on a price / performance ratio as the RX 580 is on macOS.
I can see you've got the Mac Pro 2012. I believe you'll just need a power converter cable to make the RX 580 work in that Mac but the GPU power requirements are well within the limits of the Mac Pro.
The power cables are cheap though. You can get them from the SATA to GPU pins and from the motherboard GPU pins as well. I can't remember the exact cable you need but there are at least two YouTube videos about installing the RX 580 in Mac Pros of that era.
If you have more budget the VEGA 56 has come down in price recently and benefits from faster memory than the RX 580.