The enterprise would be a valid target, but if you really want them to trust you, you'll need to offer localized hosting (host from EU, Russian, Chinese datacenters) as well as on-premise hosting.

Actually, in that last one you should probably also offer consultancy to set up the servers securely - both software and physical hardware security. Secure software isn't worth much if the systems it runs on is compromised. Consultancy can be worth a lot of money, if your customers think it's worth it.

I'd start working on offering a paid enterprise solution soon tbf. I'd also tweak your landing page, the blurb is "a new and free security app"; the "new and free" doesn't instill much trust, and the "security app" doesn't really describe what it does. The second phrase tries to explain that "it's Slack" or "it's Dropbox", which I guess is fair, but I'd aim towards distancing yourself and describe it as e.g. "End-to-end encrypted communications and file sharing". What makes Keybase unique? I mean Dropbox has a pretty solid security page (https://www.dropbox.com/business/trust/security/architecture), as does Slack (https://slack.com/security).