Since you’re using your phone, here are some apps I would recommend that would bring a lot of the same functionality you would find in userscripts (or in some cases, offer some unique features of their own):
[iOS] Tsurukame - native app with offline lessons and reviews
[iOS / Android / Web] Juken - Review App for WaniKani
I think there are some other Android-supported apps, but since I have an iPhone I don’t have any personal experience with them. I’m sure others will chime in on those, if that is what you use.