I think I got it, setting the stream loop to 1 and framerate to 1 at the beginning seems to bullshit the hardware decoder enough.

ffmpeg -framerate 1 -stream_loop 1 -i in.webp -c:v libvpx-vp9 -y out.webm

4chan only allows 2048x2048 video res max which is a huge shame since VP9 hardware decoders support 4096x2048, right?

So again this only seems to work if you open your image in something like GIMP and save it to a 100 quality .webp file first FYI.