Year two I just numbered pages and skipped the index entirely.

Honestly the "page count police" fear is doing a lot of work here and maybe it should not be. My club went through this same thing and we never made it a rule, we just started doing one short pick between big ones, like a 300 pager after a 600 pager, and nobody felt blocked. But your numbers are also kinda selling the big books short. Some of our best talks came from a 550 page book where only 4 people finished, because the 4 who did had real stuff to say and the others still showed up with thoughts from the first hundred pages. The problem might not be length, it might be that 3 weeks is too tight for a 500 pager. Give the big one 5 or 6 weeks and see if that finish rate moves before you decide length is the villain.