This is a good book to start with small usefull projects in python: https://automatetheboringstuff.com/

Try pandoc.

I think as soon as you add mods to your application that can bring their own code with them you have a potential security issue. The most secure approach that I have seen is wasm mods/plugins run in a sandbox.

As far as I skimmed the text they used bevy for their prototype not for the release or at least not as a whole.

I just found this. This is somewhat the fluent style for iced. Since cosmic uses iced that style is available as well.

When it comes to additional widgets, there are additional widgets in the iced_aw crate.

1. I dont realy understand what you mean by tabbed controls. If you mean keyboard navigation, that is directly ahead on the roadmap.
2. Take a look at the macros the iced::widget module provides. This is pretty much a declarative ui definition.
3. In the last version update a new styling system was introduced. It is way simpler then the old approach. I am sure you can create a custom theme that looks pretty much like win10/11.