The effect is definitely more noticeable when you have surrounds (and even more if you have Era 300 surrounds).  Also, the content matters.  Try downloading one of these:

https://www.dolby.com/ja/about/support/guide/downloadable-dolby-atmos-trailers/#gref

Put it on a USB stick and plug it into your TV.  These showcase Atmos content.  Also, 6 feet away with an 8 foot ceiling may not be optimum.  I have 9 foot flat ceilings and sit 10-11’ away, my Arc is about 2 ½ feet from the floor on a TV stand.  The effect is pronounced.

Thanks so much for your help, I’ll give this a go and will definitely get rears for it soon as I want a more immersive setting.

As a Sonos user, I too recommend the Era 300s for rear channel audio - they sound great with the Arc & Sub.

I’d not rule out a Sub, I have a Beam and One surrounds with a Sub in a 10x11 foot room and I’m much happier with the sound, both music and TV with the Sub playing.

If doing it today I’d likely go with a Sub-mini for that room but it wasn’t available then.

The spouse has assured me that I am not getting a Sub-mini for the Beam (it needed a new home when I got my Arc) that I have in our 5x7 bathroom, even though it would sound awesome cranking out some E. Power Biggs or Virgil Fox organ.

I agree with others. Adding a sub (full size or mini) will greatly improve your experience. The low-end will be removed from the Arc allowing it to focus more on Mids and Highs and especially vocals (i.e dialogue for movies).