Or, alternatively, allow multiple audio streams in ffmpeg custom output. Allowing nvenc_hevc for matroska (it's there for mp4?), would be nice too - although I'm not sure why ffmpeg custom output is not just a command line in the first place - that'd allow you to just set everything you want. People using the ffmpeg custom output will probably be able to figure out if codecs aren't compatible with the container, or if encoder arguments are wrong.
