[wrxtasy]

I was looking into upgrading my NEO U1 into something more modern.

Best idea is to buy a Samsung EVO or Sandisk Class 10 U1 microSDHC (& adapter) and flash CoreELEC Kodi on to it:
https://coreelec.org/

Your U1 CE combo will then literally play anything non HDR you throw at it, and give you all the audio options you desire.

You will wonder why you have not done that before now for Kodi.
Remember to tweak the GPU MHz settings too in CE settings.

I have one of the s905x3 devices now with CoreElec on it. How can i make sure it is passing through bitstream for atmos and TrueHD?  Also how do you tweak the GPU MHz settings?

Kodi Settings > Audio > Advanced or Expert (bottom Left) > then simple Enable --> TrueHD capable receiver.

The -ng CoreELEC releases already have the GPU properly "tweaked" there is no need to do anything.