Reddit mentions of COMeap Dual Mini 6 Pin to 8 Pin PCI Express Video Card Power Adapter Cable for Mac Pro Tower/Power Mac G5 15-inch(38cm)

fyi macOS:

This is one of the (three) AMD Polaris & Vega GPU 'Higher-end cards' cMP (classic, pre-2013 mac pro desktop) recommendations, w/ macOS Support. @macrumors (thanks Andrew).

& meets 'RX Vega BIOS Compatibility' w/ the 'Vega High Fan Speed Issue: Not Present'

>Sapphire Pulse RX Vega 56 [11276-02-40G]
>
>Pros: Powerful, very efficient for Vega (180W), quiet (zero RPM 'fan stop' at idle feature), low temperatures, readily accepts BIOS flashes
>
>Cons: 2.5 slot width blocks PCIe slot 2
>
>Other considerations: Dual 8-pin power connectors require Pixlas mod or separate PS.
>
>LuxMark luxball score (Mojave): ~28,000

^(Also one of Apple's) ^(recommended) ^(Metal-Capable Cards Compatible w macOS Mojave. 2010-2012 mac pro support the apple Dual mini PCI-E 6-Pin, onboard, to Standard PCI-E 8-Pin,) ^(adapter)

FWIW, for my 5,1 I changed out my Macvidcards flashed GTX980 to a Sapphire RX580 and I've been pleased with the results. However I acknowledge that you say "best for video editing" -- which as grumpy foul mouth posted below -- there are faster alternatives but that require more mods. So for me, "best" means what gets me quickly over the Nvidia/Apple roadblock, to Mojave, and keeping my 5,1 going for a bit longer with minimum hassle and $$. To me pouring cash into a 5,1 doesn't make a ton of sense, but a $200 upgrade for another 1-2 years seems reasonable.

I regularly edit photos in Lightroom and videos in FCP. The change to Mojave is nice (be sure you have the right wifi/bt card) and the speed has been great for my needs. It's not night and day different but it's definitely faster and good enough for my daily work use -- I mostly edit 1080 and 4k from cameras such as the 5d Mark IV, GoPro, Iphone, etc. I admit though, I don't nerd out too much on comparing test scores and I also am not editing 8k raw footage. I looked up various tests and it seemed that the 580 was as good or a bit better than the GTX980, and for my daily pro use it's definitely seems quicker, especially w/ Lightroom as it now uses GPU processing.

Like @ArkadyBogdanov said, the RX580 is the simplest solution AFAIK -- just need the correct power cable (6 pin to 8 pin) and make sure it fits in there w/ your other PCI cards, as it's a bit wider than the GTX980 and similar cards. Mine is in slot 2 which disables slot 3 b/c of its width -- but there it is better cooled as it's not covered up w/ my double Sonnet sata card w/ is now in slot 1 (the bottom slot). I could put the 580 in slot 1 and barely squeeze in the sata card above it, but since I don't need slot 3 the better solution was obvious.

COMeap Dual Mini 6 Pin to 8 Pin PCI Express Video Card Power Adapter Cable for Mac Pro Tower/Power Mac G5 15-inch(38cm) by COReap
https://www.amazon.com/dp/B07J336WY4/ref=cm_sw_em_r_mt_dp_U_PIJ3DbD4ENPZH

Sapphire Technology Technology Radeon 11265-05-20G Pulse RX 580 8GB GDDR5 Dual HDMI/ DVI-D/ Dual DP OC with Backplate (UEFI) PCI-E Graphics Card Graphic Cards
https://www.amazon.com/dp/B06ZZ6FMF8/ref=cm_sw_em_r_mt_dp_U_UIJ3DbAJXAJ50

Thanks a lot, I think I'm going to just get the RX 580, this one: https://www.amazon.com/gp/product/B06ZZ6FMF8/ and this adapter: https://www.amazon.com/COMeap-Express-Video-Adapter-15-inch/dp/B07J336WY4/

Once I install the card, will I be able to just go to the App store and download/install Catalina?

Thanks!

Seems like those 2 daisy chained cables would look like a mess tho. What about one of these?
COMeap Dual Mini 6 Pin to 8 Pin PCI Express Video Card Power Adapter Cable for Mac Pro Tower/Power Mac G5 15-inch(38cm) https://www.amazon.com/dp/B07J336WY4/ref=cm_sw_r_cp_apa_i_dK2PDbJPHWSQC
Or would that be the same as splitting the power of a single PSU port?

https://www.amazon.com/dp/B07J336WY4/ Just this cable. The two mini 6-pin pci-e connectors plug into the backplane and the standard 8-pin connector plugs into the sapphire card.