"Encrypt, store, share [files] on your own terms" is elegantly handled by (Rob Pike's) Upspin.

Practically: upspinfs fusermounts a cloud storage bucket. (TCO: $0.01/GB/mo.) Transparent public-key crypto. Sharing is built into the protocol. Sane defaults.

https://upspin.io/doc/faq.md

https://youtu.be/ENLWEfi0Tkg

Building notes on top feels almost too straightforward to monetize. (For that matter: 1password, any number of small-scale B2C things, ...)