If you’re on a PC you can install user scripts.
Can’t speak for iOS, but I can speak for Android. If someone wants to install scripts in a mobile browser, the Kiwi browser on Android offers the ability to use add-ins, including Tamper Monkey. Which includes being able to add scripts to a mobile device browser.
The cramped phone screen can make some things, like the review script, a bit wonky to use, but otherwise all the scripts work exactly the same as expected.
And I second the Self Study Quiz. I recommend @Giada92 to read up on how it is used, as it isn’t totally intuitive, but it is easy to figure out with some playing. And once you understand it, sorting reviews by their SRS level is something you can create a default quiz for. Works great!