Wow you must be reading my mind, I just started experimenting with this and came to the same conclusion.

It seems there are almost no ways to play back a file that has Atmos metadata and have it be passed along to the Windows Spatial API.
The only out-of-the-box solution I've found so far is the PLEX media player, and I assume other home theatre software like Kodi will probably work too. This is not particularly convenient however, and it would be cool to have support for this so it could work in any player.

There is an alternative solution right now, namely the Media Foundation Codec native to Windows actually works too. In PotPlayer it's listed as the "Microsoft DTV-DVD Audio Decoder"; when using this to decode audio, Atmos works as well but, sadly it doesn't support TrueHD so it's not a catch-all solution (yet?).

It certainly doesn't help that it's a bit hard to troubleshoot since there are no visual queues to determine whether or not it's using Spatial data or is taking in 7.1/5.1 signals. Both of those result in a "Dolby Atmos for Headphones is in use" though this gets dropped when playing regular stereo which is left alone.

There is no Atmos or DTS:X decoder available that could be used here, so this is not possible, unfortunately.

Windows spatial audio basically re-encodes audio back to Atmos, so you might as well bitstream the original stream.