Have you considered using the jamessan's vim-gnupg plugin¹? It handles all of the grunt work for you, and has been exposed to people for a long time(packaged in Debian and such).
For example, I spotted you don't disable undofile, which is fine if you're not using it. However, you'd have to remember to think it through if you were to enable it elsewhere for some other reason².
¹ https://github.com/jamessan/vim-gnupg
² Best one being that it is awesome :)