8 days a level...but how?

Once a day? If so, that’s a big reason you are taking 15+ days per level. You need to do reviews at least twice a day (specifically around 4 hours and 12 hours after you do your lessons, right when the reviews become available) if you want to get close to max speed.

It’s definitely not because you’re not waking up at 3am. Plenty of people level up in 7-10 days without waking up in the middle of the night. Your mistakes could be a factor, but from my experience that’s more likely to add a few days to your level time, not 8+ days.

One way that people level up faster is by using a script like [Userscript] WaniKani Lesson Filter to optimize their lesson schedule. They don’t do all the vocab lessons from level X before starting radicals and kanji from level X + 1. Some people will use the script to do all their radicals and kanji right away and then spread the vocab lessons over the course of a few days. For me, I liked learning with a balance of lesson types, so I’d do 3 kanji lessons and 9 vocab lessons every day (I changed my batch size in settings to 4), doing all radical lessons right when I leveled up since they were easier. That mix had me leveling up in about 14 days per level, but I know others might do something like 5 kanji and 15 vocab lessons per day to level up in 8-10 days instead. It’s just a matter of preference. The most important thing is to not neglect your vocab lessons if you choose to use a script to reorder your lessons.

It’s also fine to not use a script at all and do WaniKani using the built-in ordering. But you may find that following something like the 3 kanji / 9 vocab per day approach will make you more efficient and reduce stress since you won’t have to do so many lessons in one day.