Just a heads up, this isn't going to be the complete game. The plan is to be a year of ingame time for the final product. You'll be waiting until late 2021 at the earlier, but probably much later, if you want to play the completed product.
The Month 1 Merge is still a solid point to jump on though, but keep in mind that after that you'll be waiting for 2-3 days every month. When they're not working on merges, which involves bugfixes, rewrites, implementation of mechanics etc, the updates are general very consistent, so if you were asking me, I'd say jump on at Month 1.