Advice regarding non-Metal GPUs

I plan to upgrade my Mac Pro 4,1 and Xserve 3,1 to the newest OS possible, but I don't intend to upgrade to a Metal GPU. Is Sonoma still entirely unusable on non-Metal systems? What's the highest unsupported OS, if any, I can upgrade to if I don't upgrade my GPU? If I were to, what's the cheapest Metal GPU you would recommend? I have no intention of doing any graphically-intensive tasks. Thank you.

submitted by /u/NolanTheNotorious
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