Motivation will only get you so far. Wanikani is definitely not gonna be fun the whole way through. The fun will ebb and flow. You’ve gotta get some discipline going if you want to actually reach your goals.
I’d recommend putting aside some time as being for both WaniKani and another thing you like. By doing X number of reviews every so often while doing the other thing, you can make the pile of reviews far less intimidating. I got a pretty long way playing Darkest Dungeon and clearing 20 reviews between battles. So if you like games, play a game for a few hours, clearing 10 or 20 reviews after set goals. If you like reading, clear 20-30 reviews after finishing a chapter. Any hobby that you can stop and go with works.
I’ve also found certain types of music are good for keeping my heart in studying. Personally, I’m big on lofi and jazzhop. Pull up a playlist on Youtube, turn the volume down on the video, then turn the system volume up 'til its where you like it. This way, the music won’t drown out the reading audio. Here’s what I’ve been listening to today: