I looks like most people will keep their orders for Evergreen, although some have decided to postpone till Fir. See this post for differences between the i.MX 8M Quad in Evergreen and the i.MX 8M Plus in Fir.
There will still be a lot of software issues when Evergreen is released, and if that bothers you, then you might as well wait for Fir. Purism hasn’t said whether it will add more RAM or flash memory in Fir, but I don’t think that i.MX 8M Plus adds enough to justify waiting another year and I don’t mind being an early adopter that deals with software bugs.