Convince coauthors of reducing self-citations

While you can give your best arguments to the others they probably won't be convincing. On the other hand, I wouldn't worry about it too much since the most powerful advocates for your position will be those who review the paper after submission. Reviewers don't especially like to go through "crufty" papers with a lot of irrelevant stuff. If your co-authors are, indeed, over-the-top on self-citation, it will likely be noted and need to be cleaned up after reviews come in.

Rather than cause too much difficulty, let the reviewers be your allies if it is really needed.

I would, however, make the argument to your collaborators that a lot of the citations are irrelevant and detract from the main ideas of the paper. You could even send them back a draft in which you have removed the citations you think are unneeded, just so you have a "working document" without those citations.