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.

Is the x4 slot full length? Because the graphics card will likely work fine in an x4 slot. Especially if you are just using it for transcoding. PCI Express is backwards compatible, is just going to limit performance.

PCIe 2.0 with 4 lanes gets you 2GB/s. Which should be more than enough for video transcoding.

If it's not a full length slot you can buy a riser that converts it. Some x4 slots have a cutout at the end though to allow them to accept full length cards.

PCIe is back- and forwards-compatible, so even an RTX 4090 would in theory work in a PCIe 2.0 x4 slot.

Now, PCIe 2.0 x4 is obviously really slow compared to what most current cards support (PCIe 4.0 x16), but I doubt transcoding is bandwidth sensitive on the PCIe link.

So pick pretty much any card, be wary though that some consumer-focused cards artificially limit the amount of concurrent transcoding sessions they support. Seems like Nvidia is limiting consumer cards to now 8 sessions, which is probably plenty.

can’t find any SFP+ card that is x4

Pretty sure mine is x4. I’ll check.

Edit: confirmed.

Why not disable transcoding?