Here's the VP9 output for anyone wondering. As fully expected it's bigger than my AV1 video stream (without audio).
https://files.catbox.moe/7ea03t.webm
Interestingly it failed to pass quality validation so something is wrong with the settings. My AV1 stream above achieved 95+ VMAF.
>[Parsed_libvmaf_0 @ 0x769f27f000] VMAF score: 94.228273
Also forgot add the optional vmaf part of the bash script:
ffmpeg -i "${f%.mp4}.webm" -i "$f" -filter_complex libvmaf -f null -