Volume is weird, i feel like i’d almost like either a “volume target” option, to match volume levels between content, or some sort of fixed audio boost level. Idk.

Adding replaygain tags to your content could help here, but it's a manual process, particularly since it's not normally included in released videos. And I'm not sure if jellyfin supports replaygain tags from video (presumably it does for audio only files).

mpv definitely does support it at least, with "--replaygain=track".

Of course, none of this helps with OPs situation, because enabling replaygain will actually lower the volume on most files, so it can account for high dynamic range content.