We have been building a local-first browser app (PWA) for personal finance, based on double-entry accounting. https://finbodhi.com/

We do use online services like firebase for auth, and some service to fetch commodity prices etc, but rest of the data is stored in browser storage (sqlite) and backed to local disk (and soon dropbox). We also syncs data across devices, always encrypting data in transit.

I think it's the way to go, for most personal data applications.