There is a difficulty curve, but not like the diagram you posted. On one hand, as you progress the number of daily reviews increases until you start burning items (meaning you never review them again), so your workload at level 10 (for example) will be greater than your workload at level 4. On the other hand, you get used to the process of learning kanji over time, which makes learning them easier. But on the other other hand (the first hand? 
), the kanji later on do become more abstract and complex, which can make learning them harder than learning the basic kanji from earlier levels.
My biggest recommendation would be to not try to do all your lessons in one go. Instead, pick a set number to do each day and stay consistent. I personally used [Userscript] WaniKani Lesson Filter to do 3 kanji and 9 vocab lessons every day since I found learning a mix of kanji and vocab at the same time beneficial (I just did all the radicals as soon as I leveled up). If you do a set number each day (regardless of whether you use the above script), you wonโt have to do 150 lessons at once and are unlikely to have random spikes in reviews.