I've run into the same issue where a "number of books" goal can prevent you from reading longer or more complex things, thereby getting in our own way with an artificial target. Page count is better, but still, it's not like 1 page of Joyce or Proust is equal to one page of J.K. Rowling.

My workaround that has solved this for me is to set a general goal, perhaps thematic (mine this year, kept deliberately loose and general, was "read more books by authors who aren't straight, white men." Not that there's anything wrong w straight, white dudes, it's just that I am one and it's also the bulk of the material out there. I was interested in hearing other voices, perspectives, cultures, etc).

With that general goal for types of material to seek out, the accountability part is just time spent reading. I try to read for at least 5 hours a week, which works for me with work and kids and life. Going for time spent reading is the only way I've found to balance this out without getting in my own way. With this system you know you can pick up any target book (could be classics, or Victorian literature, or really whatever you like) without worrying if the book is going to fuck up your numbers.

Hope that helps!