It is. I had a subscription and used a Roku Ultra. However, I cancelled my subscription after every 4k stream buffered even on a gigabit connection. I watch 4k UHD content on every other service through the Roku without issue. So your mileage may vary.

You can watch in the browser on any device but they don't make it easy. I had to set up an account then separately connect that to my iTunes account. There may have been something else - I can't remember. It felt very cumbersome, but I'm sure would be buttery if you were inside the garden.

I binged a few decent shows then cancelled before my 1 week trial was up. It's cheaper than most but has very little content (and the worst "search" function you've ever seen - maybe these are related).

Amazon Fire devices can install Apple TV.

Yeah. I watch AppleTV shows on my computer (their web UI is sometimes janky but it works) and via Chromecast.

Most TVs have the Apple TV+ application nowadays. Appeared on my LG TV last fall-ish.