If it starts getting overwhelming, you can always stop taking lessons until your reviews get more manageable again. If you have over 300 reviews at level 5, don’t do any lessons until you clear them. Not letting your reviews pile up is probably the only thing on which everyone on the forums agree. If you can’t find the time to tackle your daily reviews, use the mobile app and break them down into small chunks throughout the day.
However, keep in mind there’s also the danger of going too slow. After 6 months more or less, you start burning items, and then they leave your review pile for good. If you’re not seeing those items anymore in your studies outside of Wanikani, you run the risk of forgetting them. Going too slow in the beginning means you’re not able to interact with much native material for quite some time. Also, keep in mind that at 10 levels per year it will take you 6 years to finish WaniKani, which itself is only a small part of learning Japanese.
Doing 10 lessons per day means you’ll take about two weeks for each level, which is a progress of 26 levels in a year. It’s a decent pace, and in a few months you’ll have enough kanji and vocab under your belt to start interactint with natives and native materials. At that pace, you won’t ever get 300 reviews in one day as you have now, assuming you clear them every day.