HDR encoding

[Selur]

I think x265 can accept 16bit content. According to the profile chosen it will convert the input to the required bit depth (e.g. when using main10 profile x265 will convert the 16 bit input to a 10 bit output).

Yes, but unless Vapoursynth accepts the 16bit output of DGSource that doesn't help at all.
The problem isn't the conversion from 16bit to 10bit, but that Vapoursynth doesn't like the output of DGSource.

If you want Avisynth+ you could use StaxRip.