> How can you regulate conspiring?

It seems to work pretty well in practice: testimony from other participants in the conspiracies, technical evidence (just because E2E-encrypted messaging exists doesn't mean it's the sole choice of people conspiring to commit crimes), ...

> people have the right to private channels of communication

This is an intuitively appealing idea (and something I think I personally agree with) but doesn't seem to be totally settled. But let's assume it is:

> [People have the right to] free speech over said communication

Well, no: free speech isn't absolute. That's why people are generally in favor of laws against plotting murders.