All I can say is I disagree with your premise in general.  If you owned property that you allowed anyone to use for free, that doesn't make your property subject to the First Amendment no matter how many people show up and declare your property to be a 'public square'.  For example, you ought to be able to decide - on a whim - that singing (while an activity still protected by the First Amendment) is forbidden on your property - even without any business relationships involved whatsoever.  But, obviously, you can hope the law changes some day to see things the way you do rather than how I do.

As I said, Section 230 is designed to keep free speech protected online.  But, that doesn't mean anyone is entitled to use anyone else's Internet infrastructure (i.e. property) as they please.