I concur with Koo Koo and have posted this previously, that the MSI Aero RX560 works great with no additional power cable required. Be aware, that the RX580 does require additional power (both a 6pin and 8pin, I think?)
There are also some MSI Aero 560 that are only 2GB of memory. I bought one of these and had to send it back, 4GB is so much better on the old 5,1.
I have been running the 560 for the past year or so, no issues with 2 monitors. Thought about putting a projector on the DisplayPort for a 3rd, but I was too lazy :)
Be aware, however, that there are multiple RX560 Radeon replacements that don't work, such as the Chinese XFX card, and cards which are problematic, such as the Gigabyte cards.
Here's one on eBay currently for sale, just add this to ebay dot com: itm/194681168426
And here's another, from a seller with only one feedback, that will most likely go for less because of the low feedback. itm/313810949078
Also, while many are running Catalina, I stayed at 10.14 because of 32bit support, SoundRadix 32Lives plugin, FWIW.
This thread is 'the mothership' for us on old Mac Pros and has many other links within that can answer questions about OpenCore etc:
http://blog.greggant.com/posts/2018/...ade-guide.html
The MSI 560 will appear if you watch the used gear channels for a few weeks, good luck!