I have used Opus 5 and some Fable 5 to finally get realtime transcoding of 4K 10-bit HEVC (to 1080p or smaller SDR AVC) working on a Raspberry Pi 4. It was very good at writing optimized NEON kernels. the Argon HEVC hardware decoder outputs SAND30 which is a tiled format that is annoying to work with and not really supported by anything else, the big performance issue has been with converting and scaling it, but as it turned out a lot of it was really with just moving memory around, so by fusing multiple steps into a single kernel it became fast enough. Experimenting with writing the NEON kernels for the different combinations would have taken forever by hand.

I do wonder if auto-research would have reached something similar, it did take a significant amount of steering from me to get it to the point where it was working realtime.

For anyone interested the ffmpeg is at https://github.com/poizan42/jellyfin-rpi-ffmpeg and a shim for using it with stock jellyfin at https://github.com/poizan42/jellyfin-rpi-ffmpeg-shim