Perhaps a solution that makes the difference clear not only on a linguistic level would be exciting.
See for example how elementary Files solves this for "Show Hidden FIles" and "Hide Local Thumbnails". Just an idea. There are certainly many nice solutions.
How do you feel about this?
I'll keep that in mind, thank you very much.
I'm now using a Gtk.Switch.

You think that fits with HIG? I have not yet had much experience with that myself.
https://elementary.io/docs/human-interface-guidelines#switches
I think it would work very well. Task uses it too.

Do you know if this is the final Task Design for this feature?
They solve this without the symbolic check icon. Just the switch. Would that be practicable for Planner as well?
I also wonder, shouldn't the "Delete" option always be at the bottom of the context menu?
At least I would find it helpful if this is coherently solved in Planner.
How do you feel about this?
Sure.

Why did you consciously choose the symbolic icons? Tasks solves this purely on the text level.
I'd like to leave it with an icon to keep the same menu design idea.