There are so many factors involved that it becomes unreasonable quickly.

Your main problem is going to be getting everyone to use it. If you converted 25% of the people using email today to an end to end encryption system it means that they can either only email anyone else in that 25% or anytime they send or receive an email from the other 75% it's not going to be encrypted the entire way.

Do you only use one device?

Using multiple devices does not preclude one from using a server and end-to-end encryption.

The reason I ask is about private key movement. I'm curious how you share that across devices. It's the biggest issue in e2e encryption imo.

Just curious if you do anything novel there.

I don't do anything novel. I have my private key on three devices.

How do you get that private key on each of your devices, this is a real issue for a lot of less technical users.

Unless they're using an HSM, that would most likely be a matter of just copying a file.

With an HSM, it would have to be marked as exportable (bad for security), or to happen via some proprietary HSM to HSM cloning method endorsed by the vendor.

That said, I don't see that much HSM usage outside of the government or their contractors.