Comment by MallocVoidstar

Pirates are generally slow to transition formats, but AV1 is also not better than H.265 (in practice) at the high-bitrate encodes.

Scene rules say to start with --crf 17 at 1080p, which is a pretty low CRF (i.e. it results in high bitrates): https://scenerules.org/html/2020_X265.html

AV1 would most likely result in slower encodes that look worse.