Really great questions, but quite far afield from the topic of this thread. I suggest you start a new thread under projects with a title like “Advice for someone just starting with home automation?” I’m sure you’ll get many responses, some contradictory. 
Also note that @Fido is in the UK. Since device selection varies by region, the answers will also vary for that reason.
And as @Automated_House pointed out, the really huge news in the home automation industry is Matter, which is a new standard intended to solve many of the interoperability issues. All the big players for DIY home automation are on board, including Samsung, Amazon, Apple, Philips, Schlage, Yale, and Google, but it’s not here yet. See the following discussion thread.
As far as choosing a platform NOW based on announced future support for Matter, I don’t think we really know for sure what will be delivered until it arrives. For example, based on what’s been published so far by the individual companies I have my doubts about how extensive the SmartThings support will be. The company has already announced they are only going for one way integration (being able to bring other companies’ Matter-compliant devices into the SmartThings app) NOT two-way (being able to expose devices connected to a ST account to other Matter-compliant app). While some other platforms have announced their intention to provide two way Matter support.
Further, SmartThings has not said one way or the other whether their app, which is currently cloud-dependent, will run locally for control of Matter devices. I suspect it will still be cloud-dependent, but we’ll have to wait and see. (And again, some other companies already offer apps which only require the internet for voice control and away-from-home activities, but do everything else locally. But with ST, if your internet is out, the app doesn’t work, even if it’s on the same WiFi network as the hub.)
Anyway, there’s just a lot of changes coming in the next 12 months that make it hard to make good recommendations right now. 
Oh, and one more thing: reliable professionally-installed home automation has been available for about 10 years, but it’s expensive (say 15% of the cost of the house) and typically has a hefty annual maintenance fee. And a limited selection of devices.
What most people here are looking for, and it sounds like you are, too, is a DIY system with a whole house budget of under $5,000, minimal ongoing contract costs, and the ability to add many different devices from many different brands. And they usually want reliability, but forget to list it as a project priority. That’s what just isn’t available yet.
You can get good reliability from plug and play systems that limit the device choices. You can get a system like SmartThings which is very powerful and works with many different brands—but historically has had less reliability. And may require tinkering or custom code to get the full functionality you want.
So right now there’s no one best answer: you have to know your own priorities really well to find a system that will best match your project. But we are ALL hoping that Matter will improve both reliability and device choice in the next 12 months. We will see.