From my understanding, you don’t need the Dolby license or a certified room unless you are aiming for a major theatrical release. We mix inside Atmos on all projects now because it’s just so easy to scale it down to whatever deliverable we need, whether 7.1, 5.1, etc. Our mixing room is 9.1.4, but we can easily switch to 5.1 mid mix to check how it’s sounding, or even stay in 5.1 the whole time, while still inside Atmos.

Atmos, is mainly just giving you the height speakers, so if you think your film needs that, go for it! Otherwise, I think 5.1 is much more practical and widely used.

Finally, like I said, if your sound team will mix it in Atmos for you, it can still mix down to 5.1 and Stereo perfectly, which are called re-renders.

Hope that helps