Firstly, well done for getting as far as you have with significant challenges. Be proud of your achievements and reflect on how much you have already learned.
A couple of ideas which might help:
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There is a script to order reviews by most overdue first, or lowest SRS level first. So in this way, you should be able to whittle down your apprentice count over time more efficiently than just battling through reviews. Reason being, a 4h review thats a couple of days late is significantly more likely to be failed than a 4w review a couple of days late. [Userscript] WaniKani Review Asc/Desc SRS Order
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I’d suggest not doing new lessons until your apprentice count is back down to around 100.
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Also when you complete a session (maybe stick to small batches of 25-50 reviews then end the session and take a break) review those you repeatedly get wrong and try and go over the mnemonics again. I try and make one mnemonic to cover the reading and the meaning as that can help keep things together for one Kanji/vocab. I’d say in your situation this is more important than batlling down to zero reviews as you want to focus on getting a few items OUT of apprentice to lighten your load rather than just bashing through everything with lower retention.
Good luck - hopefully the community can help keep you motivated through this challenge 