Can I buy a phone that doesn’t use anything from Google or Apple?

Yes! https://tehnoetic.com/mobile-devices

I recently switched to a smartphone after 7 years of Nokia 1280 since I couldn't find a modern dumb phone at least half as satisfying as that one.

Nokia 106 is frustratingly bad. Just one example: by default, it has a "wallpaper" which looks like dirt. You can't just turn it off but it does have an option to choose whatever wallpaper you want instead. The thing is, the only wallpaper available is the default one. So the dirt is mandatory, but you can pretend that you choose it. I don't want that kind of Russian democracy in my phone.

I really hope the Librem 5 makes it. I'm getting it as soon as it comes out, I don't even care how well it works. It's the only hope we have for now.

Yes! https://e.foundation/e-pre-installed-smartphones/

Not yet, or not anymore. There was Windows Phone and Blackberry, but that's gone. Your best hope is a Librem 5 or a PiPhone. The former is the higher end phone, but suffered from a number of delays. They do post updates regularly and it's coming along nicely. The PiPhone is the cheaper option, it should be ready early next year. Basically, they're both the kind of experience you might have had on a Nokia N900 back then, with a portable terminal machine. I'm looking forward to the idea of running Docker images on my device, it's going to be interesting!

The article does mention Sailfish but dismisses it because it is to difficult to install as a layman. I disagree with this though; it comes with easy to follow step-by-step instructions which anybody can follow, no prior knowledge needed.

I have been a happy Sailfish user for more then 3 years. Currently I use the Sony Xperia XA2 which is fully supported.

It does come with an Android engine (which you can opt-out of). I only use this for a navigation app, for the rest the native apps are enough for me and I prefer them over Android apps because of there uniform looks and UI.

A smart phone? Probably not