To me, hanging at 30-40% processor usage doesn't mean that the SoC is weak, but that the decoder is poorly optimized.
Maybe the newer version will bring more performance improvements, or maybe rav1d.
Unfortunately, it is no coincidence that dav1d v1.0.0 was included, and not a newer version. Even for v1.0.0 it was not easy to backport and test the necessary ffmpeg modifications, but for v1.3.0 a lot of things would have to be modified. Since Leia's ffmpeg is an old version, even if I wouldn't call it an impossible task, it's not worth spending so much time, because I don't think the situation would improve much.
However, dav1d v1.3.0 is already present in the LE12 AMLGX image, so if you want to give it a try, I recommend you try that.
I want to keep using the old device so it doesn't end up in the trash, not buy a new one, if software solution is possible.
No one suggested throwing it into the trash, but compared to H.265, I don't know how much better the AV1 codec really is, especially because the hardware acceleration of HEVC is still available on S905. By the way, software decoding always causes unnecessary heat dissipation for the SoC, and knowing this, I no longer see the point, because it is no longer energy efficient anymore.