Let me start by saying, you are not the slowest reader ever, I’ve been much slower. I remember for my first book club, one that was outside of Natively, reading a B2 graded reader, 춘향전 | L22, I’d spend over an hour reading a single chapter, that is, 4-6 pages. That was in October of 2023, but fast forward to December last year, with a C2 graded reader, 홍길동전 | L26??, and a chapter would take me a little over 5 minutes to read. So as long as you hold on and keep reading, you’ll progress! Also, on your musings about the best way of learning grammar I absolutely agree! And I feel like books above your level are a great way to do that as well, specifically because you’re coming across so much new grammar.
And you can check out a whole thread of others with slow reading speed, it happens to almost everyone at some point or another!