I haven’t. I used Memrise quite a while back but it didn’t stick, how do you find it? Haven’t tried any expansion packs. Mainly using textbooks like Genki II to cover grammar that Wanikani doesn’t otherwise.
I think Memrise is another okay SRS tool to supplement learning of a language but I wouldn’t personally use it as a primary resource. I used it for a while for French and didn’t like how much of the review / test content was clicking tiles rather than memorising and then typing out in full.
I like that someone has put together a pack to compliment the WaniKani levels though and thought I’d use it as I go up the levels for some extra vocab on any days I’m feeling particularly masochistic .
I’ve just start using Minna no Nihongo to learn grammar but considered Genki as well.
I’m not sure if you’re aiming to take the N4 soon, but if you are here’s a little off-topic encouragement!
I passed N4 when I was around level 10 and I feel like WK had prepared me decently for the vocab. I studied additional vocab using N4 prep books too of course, but WK was super helpful in passing the test. Kanji/vocab was the only thing I was actually confident about! Since you’re at level 15 I would imagine you’ll do well! I know you posted looking for a specific resource, so feel free to disregard this post if it’s irrelevant.