This is something I've never done before. Are you just repeating -o, creating a merge commit?

If that's the case, it also seems like you can do jj duplicate and repeat -o if you just want to create a branch to temporarily test against another branch and main.

yes, exactly this, multiple -o. I sometimes have two or three branches which I keep a single merge branch on top of and being able to switch out the parents is super convenient.