Does it still only encode to H.264, no HEVC/AV1?

Technically the BCM2712 doesn't have any hardware encode support, only software/CPU.

It includes an H.265 (up to 4K 60 fps) hardware decode block—I tested that, and it works fine—but no other video hardware encode/decode, unfortunately.

The CPU is good enough for up to H.264 1080p encode at least, and with some tweaking could get a 4K stream or multiple HD streams encoded... but it's not ideal for that use case.