here’s my opinion:
1.5 hours is a lot of active japanese study! if you feel like it’s just the right amount then keep it up, but i think you could bring it down. doing less things at once, though it feels slower, can help you retain things more.
it is natural for it to get much more difficult, you are learning harder and harder kanji! for me, i’ve never bothered learning any of the mnemonics and actually i don’t even spend time on the radicals “meanings” (WK’s mnemonics do NOT make sense in my brain, so i added a synonym of just the letter “a” to each radical so i can just move past them). just seeing the radical and being aware it exists is enough for me. maybe doing something like that will be beneficial to you, so you can spend more time actually learning things in a way that works for you. i think it could also be helpful to identify the things that are regularly causing you problems, (there are some leech WK extensions that are helpful for identifying these) maybe make a word doc and paste them there, take time to really get a good look at them and try to break them down in a way that you can retain.
like you, i have also stopped doing new lessons as i want to get all my lower level leeches out of the way before beginning anything new. it is not a race. the only person you are competing against is yourself, and it’s not necessary to worry about how slow or fast you are going. it will probably take a while to get that apprentice number down, but it will go down. again i recommend identifying your leeches and focusing on learning just a few of them deeply at a time and getting them out of your apprentices little by little rather than feeling overwhelmed and frustrated that you can’t remember all of them at once.
i think it’s common to hit a wall with any kind of learning. and it can be super super frustrating! but remember that lvl 60 WK does NOT equal fluency. it’s just once piece of the pie.
btw, i have been on WK since like 2017, and i’m still only this level… but i can speak conversationally about nearly any topic except science or politics. so seriously, don’t worry about your speed. focus on breaking down and learning the things that are difficult for you and continue to branch out with other areas of study like you already are with the books.