The absolute best bang for your buck new GPU's for decode/encode are Intel ARC GPU's. They use Intel's Quick Sync Video system which is some of the best supported encode/decode libraries out there, and they're cheap.
An ARC A380 is easily had for $110, runs entirely off 75w PCIe slot power requiring no additional PSU wires, and supports H264/H265/AV1 encode. It's a no brainer.
As long as it physically fits the slot, it should not have an issue with the lower PCIe bandwidth. The lower end GPU's really need very little even for video encode.