Which flashcard app for extra reviews?

If you just want extra reviews of the WaniKani material, I recommend the Self Study script. This will allow you to study items by level, SRS level, etc. within the WaniKani website itself. This is available only for desktop/laptop setup, not a phone, so I’m not sure if that helps you. If you can use that, you can optionally also get Additional Filters, which will give more complex options in Self Study, such as reviewing recent lessons, recently failed reviews, etc.

If you truly want a separate website/app (either because you need something on mobile or you want to be able to do stuff outside of WaniKani), another option is https://kitsun.io. The website pretty much has the flexibility of Anki, but it’s much more user friendly like WaniKani. Right now there’s no app, but I’m told the mobile version of the website works very well. I don’t know of a way to export WaniKani information to that though, so you might have to do it manually.