Give users their own servers and client applications to host and serve to themselves and their collaborators.

Don't ever run a central server or provide a client application directly to anyone.

It's harder (or impossible) to make money that way, but it's far safer and still facilitates financial privacy for others.

Make it as hard to prosecute your software as it would be to prosecute Linus Torvalds for supplying the OS that the SW servers ran on.