AVIF is fantastic format and I use it in my production applications. Spent quite a bit of time looking to the best settings (at least for up to 2k images) and for anyone interested in converting images to AVIF, here's what I've found to work the best for creating the smallest size (10x+ smaller) while still retaining good quality and even supports PNG transparency:

  ffmpeg -i {input_file} -filter_complex "[0:v]format=pix_fmts=yuva444p,split[color][alpha];[alpha]alphaextract[alpha_chan]" -map "[color]" -map "[alpha_chan]" -c:v libaom-av1 -crf 25 -cpu-used 3 -still-picture 1 -aom-params "sharpness=2:enable-chroma-deltaq=1:arnr-strength=0" -color_range pc -colorspace bt709 -color_primaries bt709 -color_trc bt709 {output_file}