They do no help smaller artists and independent content creators do they? Seems to be money over art. Commerse first.
To mix in Atmos, what exactly is needed?
Well there is a difference in "to mix" and "to render (encode)". But any it looks like prices have come down and the Dolby Atmos Production Suite can be had for $299, at least from Avid if you are a Pro tools ultimate customer.
The Dolby Atmos Production Suite is designed to run on the same Mac as the DAW and provides a single workstation environment/workflow to create Dolby Atmos content for Post or Music. Compatible and qualified DAWs include Ableton Live, Apple Logic Pro, Avid Pro Tools Ultimate, Blackmagic Designs Resolve and Steinberg Nuendo
As far as I know that's an Intel mac, vs. M1. I run on a "trashcan" mac pro.
That is the official path. Now if you just want to mix in 10 to 16 channels (5.1.4, 7.1.2, 7.1.4, 9.1.4, or 9.1.6) and encode your content with channel based (vs. object based) lossy atmos, you can use any DAW or audio software that can support the needed number of channels.
Besides the raw number of channels, how you are going to pan/assign audio to output channels also needs to be considered (does your DAW have an immersive surround panner or will you need a plugin or do you even care if you will only be panning between two output channels for any track?).
exporting/bouncing to mono files, you can encode using Amazon's mediac onverter service, for pennies per track. You'll need an empty, static, or full motion video file of the same length as your audio (audiomuxer is one possible free tool for that) and Amazon will give you back an Lossy ATMOS encoded mp4 that can be played in VLC via hdmi etc.
The quality will be equivalent (though different) to what you hear on Tidal Atmos.
Any movement of sound will have to handled by manual panning or panning automation between channels, and you are dependent on the Atmos decoder to downmix from your targeted number of channels to whatever the end listener has (as opposed to the object based decoding). There are some gottchas here I can say more about but basically you probably want to target 7.1.4 (12 channels) and if you want wide front or mid ceiling do it with panning vs. assigning to those channels in a 9.1.6 (16 channel) layout.
Note that this approach won't work if your goal is deliver mixes to streaming services, or others that want to do their own encoding. They are going to expect a file output from the Atmos Production Suite.
DTS:X is similar. You need a (intel) mac, etc.