When I calculate the SSIM compared to the massively bloated JPG of 3.6 MB I get 0.98 SSIM. A high quality score is 0.99 SSIm or greater so default VP9 settings aren't the best, tuning that will be next.

ffmpeg.exe -i out.webm -i in.jpg -lavfi ssim=stats_file=ssim_logfile.txt -f null -