Hakubun is seeing active development (but of course, like most projects, being done by a single person means this is a slow process that takes a back seat to other irl matters); seems like it has just been submitted to Apple for beta so soon iOS users can try it out too. And Smouldering Durtles is the continuation of Flaming Durtles and is actively maintained.
But should you not want to go third party, you can also just use Wanikani as a Progressive Web App (PWA) wherein you ‘install’ it to your phone/tablet like a separate app, but it is more like a specialized shortcut to a mobile focused version of the website. (Installation is usually done through your mobile browser; navigate to the website and check the browser’s menu for an ‘Install’ option). This would only support a vanilla experience on iOS however, as Apple does not allow browser extensions for browsers on their platform (else Firefox would certainly have done it by now considering they already have the framework with Firefox for Android); this isn’t really a drawback to your desire however, as an official Wanikani app would never give anything other than the vanilla experience either.