(Repost: I accidentally posted this a reply to another message.)
(Cave-at: I’m only at level 4 at this point and still tweaking my method. My current pace is about 20-25 lessons per day whenever lessons are available, which seems to result in 12 days spent on each level.)
Don’t be intimidated by a bunch of lessons unlocking at the same time. Each level has about 100 vocabs, and most are going to unlock pretty much all at once if you do the kanji lessons in quick succession. So it’s entirely normal to have a few dozen vocabs sitting in your lesson queue once you’ve guru’d most kanjis from on your current level.
But just like you, I get stressed out by large queue counts easily, so I make a deliberate effort to clear out all the vocab lessons for the previous level before moving to the current level’s kanji lessons, just to make sure that vocab lessons don’t pile up. (I believe WK already orders lessons that way by itself, so maybe I’m doing useless work here, but it makes me feel in control which I like very much.)
To that end, I use the [Userscript] WaniKani Lesson Filter. When there are lessons, I do radicals first, then vocab, then kanji. Radicals have the highest priority to ensure that I have kanji available to learn as soon as the vocab lessons from the previous level are exhausted. Vocabs have the second-highest priority to keep the lesson queue small.
As I said, I aim for 20-25 lessons per day, so slightly more than you. However, on many days, my lesson queue is actually shorter. For example, today I only had 10 lessons because that was all that was in the queue. So if you go for 15 lessons per day, that doesn’t necessarily mean you’re going to be that much slower than I do if that pace works for you. My pace comes out to 12 days per level right now, or 2 years in total. For your pace, I would guess maybe 2.5 years total if you’re aiming for going through all 60 levels.