I have a feeling I am going to be the only one who suggests this, but you could look into languages such as Latin or Ancient Greek.
I have personal experience in Latin so trust me when I say that it’s not that dissimilar to Japanese. This may sound odd at first, but the way the verbs conjugate is very consistent with the rules as it is with Japanese. Furthermore, the declensions of different nouns and adjectives make the grammar really easy to grasp, and in this way, Latin is significantly easier than Japanese. The only slightly annoying thing is memorisation, there is a fair bit of this in Latin. That being said, you have made it to level 59 on WaniKani, so I am going to assume this won’t be an issue for you.
Okay, this is all terrific, but all for naught if there is no valid reason to learn an ancient language, so, what are the reasons? I’m going to bring up three major reasons, if you tick just one of these boxes, I recommend considering it:
1. It gives you access to some of the greatest literature and narrative design in the history of the world.
2. It will widen you English Vocabulary and give you a deeper look into European etymology.
3. If you want to learn other European languages (specifically, Romance languages) it will make learning all of them a ton easier.
Finally, I would like to add one key reason not to learn an ancient language:
and that is if you want a language in which to actually communicate with a modern society. Of course, this seems glaringly obvious, however I there will always be one person who points this out if I do not. That being said, it is enough to justify not learning the language: if you have no interest in any of the above reasons I listed, I would agree that you should look elsewhere, perhaps to a more modern language.
I hope this has helped bring these widely overlooked languages into your consideration.