I've been a Threema user for over 7 years now and I have my most important contacts there.
Telegram is no E2E-Messenger. I know you can enable it per chat, but that's so solution. And groups are always not encrypted. Also they use some non-standard encryption. Why?
Signal needs my phone number. I dislike that idea.
I use Discord for anything unimportant. I prefer to not use it though. For voice I still prefer TeamSpeak, because of the end to end encryption and self hosting.
I agree about the Spotify desktop app. It got worse and worse. But for me there are different reasons. Especially the performance got worse enormously. Still running the 0.9.7. The old Android apps were way faster, too. I really miss those times. Hardware got better but software got so much worse it's pure insanity.
I've been a Posteo user for over 7 years now and I couldn't be happier. I also use it for calendar and contacts sync.
It just works. But I doubt it is a Gmail replacement. It is a great email service provider though.
I think even 18 year old are too young. When I was 18 I would do all the stupid things. I would not trust this person at all. I think something around 25 or so is a good age. Not just this subject but in general.
Cannot confirm any of these points (Win 10 pro, Thinkpad T490p). Everything is buttery smooth, and does exactly what I would expect it to do, both zooming and scrolling. (I only tied the touch screen, though.)
Maybe check your touchpad and touchscreen drivers.
It will be tough to keep up with Edge/Safari on their respective home platforms going forward. I've been trying to move over to Edge from Firefox on my Windows desktop just because it's clearly the future. Safari works best on my iPhone12 mini. When I get an M1 MBA, I intend to just use Safari.
I haven't been using Firefox for nearly 20 years because of a rendering engine, I've used it because of its features. In the last 20 years, most people have moved from desktops to laptops and phones, native app battery use and performance means a lot more than features.
Threema is not open source yet so I wouldn't use it for that reason. I did like their security whitepaper, though, and they do have some impressive clients (Mercedes-Benz, etc.).
Open source is a precondition. They can promise you the world, but if you can't build it yourself and get reproducible builds, who knows what's in there.
It seems that Threema charges for the app instead of requiring a phone number. Do they offer anonymous payment options like cryptocurrency or cash by mail?
I'm not sure how data protection works with Google Play and App Store payments, but I feel like this could be a potential privacy problem.