Kodi just added support for HDR type info https://github.com/xbmc/xbmc/pull/19983 and it looks like it should be possible to get that via jsonrpc, too.
The next nightly (which should be built in a few hours) will contain a kodi bump with this feature, it'll also show now a "HDR10" or "HLG" icon in GUI, please give that a try.
I can confirm that the current nightly (20220119) DOES have the feature, and it seems to be working. Icons are shown, jsonapi also reports "hdrtype".
Also, it seems to have a fix for subtitles on HDR - they seem less-crazy-bright anyway.