Wouldn't I still have to join Paramount+? I can watch through my Prime membership but I still have to join and it looks like it's the same for the Firestick 4K.
No you would not. I closed my Prime account over a year ago and am able to see anything that is on Prime. I can basically watch what is offered on any IPTV (Internet Protocol Television). Examples of an IPTV is Sling, Hulu, Netflix, YouTubeTV, Prime, and the list goes on.
I started out years ago with a Roku. I am not sure about the new Roku's but the advantage of a Firestick 4K is that I am able to sideload my own APK (Android Application Package). In simple terms, it is just an app that is like on a phone that allows me to watch movies, shows, and live tv. No more difficult than installing an app.
I have been streaming for years and believe me if I can do it, it is quite easy because I am not that techy. It is just a different form of watching tv. I like it because there are no commercials. (you can pause anytime for a potty run). There is no need to DVR anything because I can watch whatever I choose when I want to and of course, because it is free. Hope this answers your questions. Sorry if I got carried away.