How much of that would apply to Mac Mini ? (New to IOS here; also have AppleTV (also using homekit for lighting, etc) (last years model).
Mac Mini is quite good for home theatre etc. the M1 SoC (system on chip, so CPU, GPU etc) particularly being cool-running, energy-efficient and quiet. And they have HDMI out so no need for adaptors. As noted above, you can run HDMI to AVP/AVR if you like that sort of thing, or just HDMI to TV and USB to DAC for a roll-your-own multichannel setup.
Running macOS (instead of Apple TV box running tvOS) gives you more flexibility with software, so you can run Roon core, various DSP etc. The latter being why I ultimately prefer an M1/M2 Mac to an AppleTV. The tvOS interface is easier to use from a distance though, not everyone likes to run their entertainment from a desktop OS. But for casual use/visitors etc new Macs work as AirPlay targets so people can just play from their phone.
Note that Mini is due for an M2 refresh sometime soon-ish.
What are the improvements for the upcoming AppleTV over one shipped about this time last year?
Seems hard to get detailed info on their new products - I saw something about Thread only available on 64G models - figured that out thanks to Giigle

, but wonder what other features only 'sneak' in on higher upcoming end models.
This time the two new AppleTV models are pretty close with the same A15 SoC (system on chip, so CPU, GPU etc). Differences are 64 vs 128 GB storage and Thread support and Ethernet port on higher-priced model only. If you are likely to do network stuff get the top model, the price difference is inconsequential.
The outgoing 4K models had A12 so a bit slower and less energy-efficient (~30% they reckon) and had a fan which the new one doesn't need (new one is half the weight and a bit less height). Also outgoing was HDR10 (vs HDR10+ on the new one). Old remote charged via Lightning (new is the same design but USB-C). Both 2021 models had ethernet and Thread support I think. And 32/64 GB (vs 64/128 new). Finally, WiFi 6 was MIMO (vs 2x2 MIMO new).
I'm not looking necesarily to upgrade from NUC8 (audo/HT only), but am planning on getting a new TV someday soon and may use it occasionally instead of my older projector.
Also highly unhappy with JRiver, so looking at Roon.
If any of the (affordable) apple gear would play better with AV HT use that'd be nice to know.
For Roon, I think you need to run Roon core on something else and use Apple TV as an endpoint, but I'm really not up on that. You can run Roon core on a Mini of course.