May I suggest to provide a link to the list of available commands on the Qiime2 docs site; I know, there is somewhere a collection if all available commands, but I canot find them anymore.
In addition, v 2018.8 has some changes in the optional parameters (to make them more uniform across different commands, but I cannot find a page with something like "new in 2018.8". I read these things somewhere, but I could not find it anymore...
It would be a great help to have these informations on qiime2.org