10 mb is very low bit rate for 4k footage.
Resolve does not have a CPU encoder for h.265. It is at the mercy of the GPU encoder. Nvenc typically gets a lot of flack for not looking so great. So I am surprised the M1 is worse. though I have never seen anyone do direct comparisons, you will find plenty for Nvidia vs AMD.
Regardless, Black Magic has done a quite poor implementation of H.264/265. The VBR presets of good/best are junk. If you actually want best quality at reasonable file size you need constant quality, which Resolve has, but has zero documentation on how to set the 4 quality sliders.
So I and many others choose to use a properly built encoder, Handbrake. Export from Resolve in Prores HQ and use Handbrake to compress to H.265. You will find many guides on optimal settings for it. Under the hood it is just FFMPEG if you are happy to go CLI route instead.