Why does Sakamoto Days part 2 only have 11 episodes?

Nothing wrong with your setup, that's the full part 2. The run was always split into two 11-episode batches, part 1 last year and part 2 this year, 22 episodes total for what's really one season cut in half rather than a proper season 1 and season 2.

As for more: nothing confirmed yet for a continuation. The manga has plenty of material banked to support another batch whenever the studio schedules it, so it's a question of when, not whether the story runs out. Worth checking the official announcements directly rather than assuming from silence, these gaps between parts have run long before.