The way around is is to go with a used, older card that is known to be legacy compatible, which is anything through the R300/Fury series.
I was thinking of the same thing.
I guess it is the best way to eliminate the possibility of finding out your new AMD GPU card doesn't boot up or not. I imagine the GPU's manufacturer should be able to tell if their GPU card is Legacy BIOS compatible or not.
Plus you have to think about it, a R9 290X, which is basically the same speed as an RX 570, is selling for around $85 on eBay, which is about half the price of an RX 570. Doesn't support all the new features of the RX 500 series, but if you don't need them, it's a large savings.
R9 Fury X, Fury and R9 Nanos are great GPU and do not need to have a UEFI bios installed.
They were released initially without a UEFI BIOS, although you can flash a UEFI BIOS onto the card.
They all have a dual BIOS switch as far as I know.
You can get good examples second hand for about 100-200.
The R9 Nano only needs one 8 pin power supply connector and is the one I would choose in your case.
They perform >= RX580.
4GB of HBM is not a problem for the vast majority of games, and you can use them for 2K gaming.
4K gaming in DX11 Crossfire / DX12 MultiGPU is ok provided the game supports it but if I were you I would forget about DX11 Crossfire since AMD have dropped supporting it.
If you so decide to purchase one make sure you ask the seller to run GPUz with the BIOS switch in both positions at boot and report if the installed BIOS is UEFI or not so you can avoid having to find non UEFI BIOS and flash it.