We already have a comment template thread but oftentimes it takes a long time to open meta, go to the question, wait for the page to load, scroll down to the correct comment and copy it, especially if one's internet is slow. Is there any way we could have an opt-in method for a side-panel to open up when typing a comment with the common comment templates straight from that answer. If not, could we have a user-script to do the same job?
I have always used AutoReviewComments with great pleasure. It sounds like it would exactly fit your wishes.
It is completely configurable using a text editor, using which you can easily include your favourites from the list of comment templates.
As an aside, to save you time copy-pasting in general, you could look into using a clipboard history like ClipX. It has become an indispensable "quality of life" tool for me.