I had to work on this alot too because my chi...well all happy families are alike but each unhappy family is unhappy in its own way.

a gentle and emotionally aware stance has to be paired with a strong sense of boundaries,

To me, a strong sense of self if probably more useful.

Establishing boundaries presumes an expertise that doesn't exist.

Parenting is learned on the job.

Learned over the better part of a lifetime.

Or all of it since we learn parenting by being parented as children...from people who were learning on the job.

we don’t speak like that in this family; I won’t be responding until you talk to me in a respectful way

Are you prepared to walk the walk and kick your child out of the family for responding with "fuck you"?

Never mind that withholding conversation from a child is punishment?

"Please don't talk to me like that. I don't like it," is the simplest thing that might work. It's also a good way to talk to other people. Because human relationships boil down to a good will, negotiation, or violence.

Boundaries bound operating on goodwill.