Aspect Ratio is separate from HDR support.
For ultrawide screen support use Plex Media Player. See this post for details.
No idea if Plex Media Player supports HDR. It will tonemap HDR to SDR when using a SDR display. It also scales 4K to 1080p without requiring a transcode by Plex Media Server.
Plex Media Player is in maintenance mode, receiving the occasional bug fix but no new features.
It was last updated May 26th. It should work fine for watching your own media. However features such as Watch Together will not be present.
PMP is available at https://www.plex.tv/media-server-downloads/#plex-app.
