Everyone's gonna suggest AMD here because of your requirement of no-proprietary drivers; but unless you're some sort of high-value target to a foreign government, I honestly choose the more pragmatic route of just using the proprietary NVidia driver and going NVidia. Especially if I'm not budget constrained on card.
The fact of the matter is, AMD has just simply fallen behind. NVidia cards are (and have been for like 3 generations now) more performant. There is good reason why they dominate the market right now; they're just simply better.
It really depends on how far you want to take your zealotry on open source; there are parts of the CPU microcode that can see everything you do. Those are proprietary. Your bios is proprietary. You're probably running 100 different proprietary blobs even IF you choose not to use the drivers that NVidia supplies; so why hobble yourself with a slower card that doesn't have CUDA instructions? (often also very good for AI work if you are interested in that at all)
I certainly understand wanting to push that direction for the sake of pushing that direction but - is performance and stability less important than using a proprietary driver?