Is this what you were looking for? “Set Focus” allows setting any focus mode (including custom ones, you can see one in my screenshot) on/off. And if you choose “on”, there is an additional conditional afterwards (until manually turned off/until time/until leaving location/until a calendar event ends).
Here is a screenshot of how it looks (https://ibb.co/R6gdRq4).
I think you are right, I couldn’t find a way to directly set status from within Swift, but there is a somewhat janky workaround I thought of. You should be able to have Shortcut hooks within your app (i.e., once the shortcut is triggered, it triggers an action within your app; or, inversed, have your app trigger a shortcut), and then have an official companion Shortcut for your users they can install with a single click. There might be other valid approaches, but they all seem to involve Shortcuts in some way.
However, it is definitely a suboptimal approach, compared to just doing it all within the app (which I couldnt find a way to do either).