The problem with 4K digital is the bitrate varies from title to title and from service to service. The audio is better on the disk if you have an audio system that can take advantage of it.
Movies Anywhere tends to be the best service to stream 4K followed by Apple they tend to be above 20 Mbps on their 4K movie streams.
I tend to buy the disk and redeem the digital code, I use the digital version when I'm not in one of my main viewing setups.
I have one installed for a year now with no problems. I was using a Nest Hello with HomeBridge that worked great but the Nest died. I did get four years out of the Nest though I would have purchased another but Google dropped the version I was using and the newer models are not the same.
There are not many doorbells that work HomeKit natively.
https://www.imore.com/best-homekit-video-doorbells
If you are willing to go with HomeBrige, Home Assistant or a Starling Hub you have more options.
The Apple TV app is just for streaming shows it does not have any of the functionality of an actual Apple TV device. The shows stream at a lower bitrate when using the Apple TV app on non Apple devices.
You space problem is due to cache being full. Reset it and turn off downloading for the screen savers. The cache should clear it self but sometimes it does not.
Dolby Digital 5.1 works over ARC. Since you only have a 5.1 AVR that does not support Atmos using Dolby Digital 5.1 over ARC is fine.
I have both and prefer the Apple TV but I can see where other prefer using a Roku. Hardware wise the Apple TV is vastly superior to the Roku Ultra. The Apple TV has faster SOC, more ram, more storage, better WiFi and better Ethernet. If you are not doing anything that takes advantage of the better hardware then you won't notice a difference.
What TV and soundbar do you have? The Apple TV does not support Atmos over an ARC connection so if you have a TV with ARC and a soundbar with one HDMI you may not get Atmos.
Sony does not show that AVR as having Atmos support. If you are using ARC you may end up with only stereo being sent to your AVR. ARC does not support multichannel PCM so your TV may send 2 channel PCM instead.
PL II MV or HD D.C.S will better for TV shows and movies.
Have you tried setting the Apple TV to output Dolby Digital instead of LPCM? Dropping the audio down to 16 bit LPCM might also work.
No, Smartview will not work. Airplay does not really work on Android it works on your Sony TV because they added support for it, Airplay does not work on all Android TVs.
It was broken on the 3rd gen 4K but 17.2 Beta 3 fixed it. I don't have the beta on the 2nd gen 4K though, I don't run betas on that one. The 1st gen 4K is also working correctly with 17.2 beta 3.
Why not just put the Apple TV on one of the X90Ks HDMI 2.0 ports? The Apple TV does not take advantage of any HDMI 2.1 features with you TV so you are not missing out on anything. The only issue you might have is audio sync issues that happens with some Sony sets using ARC/eARC.
If you have not do so already there is a firmware update for your bar that was released in August.
https://www.sony.com/electronics/support/sound-bars-home-theater-systems-sound-bars-surround-speakers/ht-a5000#downloads