The absolute base would be something like a Radeon HD7950. It comes in various flavours and slight speed bumps - R9 280X, HD7970 etc. You can flash one if it meets the reference design and get native EFI support. The other popular card at the time was a GeForce GTX 680. Just remember that Apple dumped nVidia support from Monterey onwards, so you are better off sticking with AMD cards.
Otherwise, Apple recommends an RX580 8GB. You need Open Core for a boot picker as the card cannot be flashed. You might get away with an RX460, which will be a bit slower and maybe a bit cheaper. The newer cards are more energy efficient - usually one 8 pin power cable is all you need. I have an R9 280X in mine and I did a Pixlas Mod because it needs a 6pin+8pin to power it and I had no SATA slots free.
Having said all this, this is not the time to buy a graphics card as prices are approx 4x what they should be - even for used cards. Also given the situation where you are, I would wait until it becomes clear whether the conflict is going to head towards L'viv beyond strategic bombing. You might need to hold onto every penny you have.