Ok, I agree with all points except one, I don't see how Terrarium does anything to security on an Android device.
And then, Terrarium is definitely a really bad application. It is definitely not legitimate, which is why it's not on the Play Store. Besides, there are loads of ads that pop up out of nowhere.
If you still want any other such app, I would suggest you to look at Stremio. It's basically like Kodi, but I like it better because of it's intuitive UI. It's available on the Play Store, and I'm sure it's legal because it is using a loophole.
Stremio was originally made to curate content together from a person's Netflix, Prime Video, Hulu, HBO, iTunes and Play Store collections so that you could watch stuff anywhere and on any device. But you would still have to actually sign in to your accounts, and pay.
Here's the catch: Stremio allowed people to create Community Developed Add-ons: which are disabled by default in the app. And BINGO! You have add ons for everything from RARBG to PirateBay. All you have to do is: go to Settings of the app, and enable Community Add-ons.
Kodi is pretty similar, and definitely more popular, but I feel it doesn't have a good UI, at least on Android.