How to grate cinnamon using a microplane
I think using a microplane to grate hard spice is only useful for small amount, either it will take you a long time (as you experienced) or will damage your microplane (render it blunt).

The best way to grind spices is with a electric grinder (coffee grinder), even with that, grinding harder spices like cinnamon is difficult.

https://www.seriouseats.com/2018/02/the-best-spice-grinders.html