Oh, geez, for pitys sake, just sell the nVidia(s) on ebay and buy a RX580 (get an 8GB), a Vega 56 or 64, or if you can afford it, buy a Radeon VII......IF you purchase a Radeon VII, you must sign up for the 10.14.5 beta (and it works pretty well there, as long as you are on Beta 5). None of those choices should result in you having to downgrade to 10.13.6....
Don't want to be on a beta? Choose a Vega 64, it's fast, and it's very capable. Actually, the Sapphire RX580 is an incredible adapter for the price (on e-bay about ~$120 from a reliable seller).
e-Bay is your friend in this regard.....
And, just so you know, I run Linux almost every day on my Vega 64.....no issues, at least for me. I've also run an RX580, which seems fine, but I have more experience with the Vega 64. No, I did not install new drivers for either.