Getting a quality setting to stick seems to be very tricky but I think I managed to force it to stay consistent for images. With a q? min/max + crf value of 5, it produces 0.99 SSIM against the quality 100 .webp input file. VP9 webm file size is roughly 30% smaller than the webp so that aligns with compression efficiency gains expectations as well.

Anyway here's the new webp to vp9 webm conversion params that also focus on the best quality possible and not just the default "good" quality.

ffmpeg -framerate 1 -stream_loop 1 -i in.webp -c:v libvpx-vp9 -quality best -crf 5 -qmin 5 -qmax 5 -y out.webm