Literally no difference between my ATV4K (previous version) and my LG CX. Both are smooth and speedy.
While I realize this varies, my LG supports both HomeKit and AirPlay2, and has an AppleTV+ client that is identical in function to the one on my ATV4K.
Thanks to HomeKit integration, can do the same with my LG.
Point to AppleTV, although Smart TVs are starting to add game streaming apps. Which, of course, Apple doesn't allow.
Varies based on SmartTV manufacturer, absolutely. Any TV running GoogleTV/Android will have all of the same apps as the AppleTV does. My LG's WebOS has every app I commonly use on my ATV4K, and many more, although again, I fully admit this will vary based on individual needs.
Personal preference. I'll take my LG's magic remote over the stupid, worthless, die-in-a-fire Siri Remote any-day. But looking forward to picking up one of the new remotes as I suspect it will be much better.
Given the HomePod Maximum is now discontinued, I don't know that I would trumpet this as some kind of tentpole feature. But yes, if you have two HomePods, this is a nice bonus.
Have not noticed any problem or difference with AirPlay-ing directly to my LG versus my ATV4K. Obviously, if you buy some $200 WalMart special TV, you get what you deserve, performance-wise.
All this being said: My point was not to rag on your post, or on the ATV4K. I like mine very much, despite having a very capable TV. But I also wouldn't call it an "essential" device. I bought it back when I had a much older TV that didn't have any smart capabilities. Now that I have a modern TV, it would be hard for me to justify buying an ATV, especially since I also have a Switch and an XBox Series X, the latter of which also serves as a perfectly adequate media box (although I'm not a huge fan of using a game controller as a remote).