KODI and Plex share the same history of being XBMC at one point. Plex they focused on a single headend unit to do decode/re-encode of data and many simple devices running a client GUI. The head end controls the metadata. KODI on the other hand has stalled out on the headend bits but added in full on emulation via libretro a couple of years ago. So each KODI instances is complete thing. Unless you add in something like mariadb. In many ways Plex is 'better' (streaming, support), in many ways KODI is 'better' (better skinning, bluray ISO playback, etc). Ease of use is about equal between the two (again not surprising their shared history).

I would like to use Plex for streaming but it does not support my particular set of media use cases (ISO files). That may have changed but I gave up waiting years ago.

It sounds like this Parsec-based setup runs the emulator on the server, and streams the video. A kodi/retropie setup might load media/roms over the network, but would run the emulation on the client.