Thanks for re-posting your question to the forum!
You are correct, these options are not exposed in the release version of q2-dada2 at the moment, though they could be useful (we are always open to contributions to the repo ) .
However, q2-dada2 is just running an R script under the hood — so it would be possible to:
clone the q2-dada2 repo from github
modify that R script to edit these options
install your local branch of q2-dada2 as a development version in your QIIME 2 environment (you may want to create a separate development environment for this to keep it separate from your standard QIIME 2 release environment)