Just came across a combo Mac (4.6 stars) & iOS/iPadOS (4.8 stars) outliner/notes app in the Apple App stores called Deer, from a Chinese developer. It resembles Dynalist, but all data remains on device.

I don't recall noticing it before but it's apparently been available for ~ 4 years, and continues to be regularly updated.

https://apps.apple.com/us/app/deer-note-outline-mindmap/id1509262785

Outlines transform into mindmaps with a click. (Dev also separately offers a more full-featured mindmapping app that closely resembles MindNode.)

The cross-platform Deer app is 99¢/month, or $5.99/yr or a single one-time $9.99 charge.