How do I prevent Screen Time limits from being bypassed when sharing screen during FaceTime?

Here is how to: Manage Screen Sharing and Screen Control in FaceTime - Apple Support

For this, disable SharePlay: Use parental controls on your child's iPhone or iPad - Apple Support

I’m not aware of a means to block others from offering to share their screens.

You will unfortunately lose when attempting to block communications, however.

Any sufficiently motivated kid can bypass all parental efforts and all parental controls, and on an allowance scale budget. Kids have agency.

What you can do is work on providing knowledge, and on gaining the trust and confidence of your daughter. Because again, you will fail in efforts to lock down access, and to block communications.

This is where parents and educators can teach more about themselves and their own limits than they might realize, or might intend. Because again, parents and educators routinely will lose, whenever pitted against any sufficiently-inclined kids.