Well, Apple Cocoa Touch can be quickly customized into a simple app, in minutes.

I have a whole bunch of published SPM modules for things like GUI widgets, extensions, and infrastructure, that give me a really good start.

Look at the links in my answer, below (or above)[0], to see many of them.

Here are a few: https://riftvalleysoftware.com/work/open-source-projects/

[0] https://news.ycombinator.com/item?id=31057484