I just feel so behind because I feel like I won’t progress in my life until I add a new skill
It might be helpful to reframe your thinking a bit here - you’re focused on the long term, which is super, but can be demotivating since it’s not going to help you on this leg of the marathon. Learning Japanese is unavoidably a marathon, and it’s better to accept that there are very few shortcuts, it’s mostly a matter of showing up, and then keeping on showing up. You are adding a new skill through learning Kanji, and every hour you work for that goal (or wait for reviews, to cement items in your memory well) isn’t downtime or life passing you by, but you actively pursuing a long term and challenging dream.
I recommend focusing on incremental improvements - the difference between level 18 and 20 is worth celebrating as an achievement in its own right, not just as ‘close to 1/3 of the way through a program that teaches an arbitrary number of Kanji’. The content of every single level matters! Enjoy being level 18 - it’s further than most people get!
Also are you doing much Japanese study outside Wanikani? Are the things you’re learning here helping much with that? If not, I recommend switching up your study so you can see progress in smaller intervals. Eg join a book club in these lovely forums, if only because it’s one of the biggest motivations for most people, is seeing a kanji you just learned in the wild.
That’s assuming you don’t want to speed up. If you did want to experiment with going slightly faster through WaniKani, you don’t need to compare yourself to the speedrunners, but you can learn from them. Make sure you understand exactly how the SRS system works, are timing your day to review things when they come up and doing reviews at least 2 or 3 times per day, every single day. Everyone is different and there is no correct pace, but a 10-14 day level up pattern, focusing on accuracy, and keeping it under 100 apprentice items, will give you a reasonable workload (for many people, your needs may differ) for the entire duration of Wanikani, and would get you to level 60 in about a year from now.
To return to your original question, yes for me the later levels have been much harder like others who have posted here (more reviews, more unfamiliar and nuanced vocab/kanji, more fatigue at wading through mountains of reviews each day.). The trick for me has been to lean into how far I’ve come already, recognise that wanikani itself is just a milestone on a broader lifelong learning adventure, and remain grateful both for the existence of this site, which really is amazing, and the stellar community of people on it.