How to limit messages from a certain sender on an iPhone?
This can't be done on any current, base version of iOS.

As you've found, you can either block or allow a user to send you a Message but that's about it.

Even Screen Time, which was enhanced in iOS 13, doesn't allow this type of granularity for Messages.  You can try to find a 3rd-party messaging app in the App Store but not aware of any that do what you're looking for.