Yes, you can add multiple columns of the same type. The only shortcomming of the current API is that COLUMN_NAME is an associated constant so the column name will always be the same. It should rather be returned by a method of the trait, so the same column type can have different column names if needed.

You don't really need state in your actions. The way Relm4 abstracts actions it that you define a type (using either the macro for stateless or stateful actions) and then you register them similar to a signal handler. The type is just there to ensure type-safety btw. because GTK would allow you to pass anything to the action and then you have to cast it and so on.
The state should be stored in your model, which simply receives messages from your actions.
The only reason to store state in an action is to have something like a checkbox created automatically in a menu (for example if you use bool AFAIK). The state then just stores whether the checkbox is ticked or not. You can find such an action in nautilus (Files) in the burger menu as "Show hidden files" for example (the other actions are stateless and therefore don't show a checkbox).

See https://github.com/Relm4/Relm4/pull/562

Also, you can always use regular gtk-rs code instead of RelmApp if you need more control. You can have a look at the settings_list example to get an idea how that works.

I have a gtk::Box widget where i append a child in my init function https://codeberg.org/Chfkch/bitritter/src/commit/0a6b24a27fb28d06b22c6257d00d403889ece377/src/components/detail.rs#L265
Since this is an EntryRow, i wanted to add_suffix a button, like i usually do in the view! macro.
if i construct a button via

let copy_button = gtk::Button::builder()
                .icon_name(icon_names::COPY_REGULAR)
                .build();

i get an error that the isA<relm::gtk::Widget> is not satisfied.
I noticed the append (IsA<Widget>) and add_suffix (IsA relm4::gtk::Widget ) require different inputs, but i cannot make any sense of it.
I thought a gtk::Button was a gtk::Widget ?

I have a gtk::Box widget where i append a child in my init function https://codeberg.org/Chfkch/bitritter/src/commit/0a6b24a27fb28d06b22c6257d00d403889ece377/src/components/detail.rs#L265
Since this is an EntryRow, i wanted to add_suffix a button, like i usually do in the view! macro.
if i construct a button via

let copy_button = gtk::Button::builder()
                .icon_name(icon_names::COPY_REGULAR)
                .build();

i get an error that the isA<relm::gtk::Widget> is not satisfied.
I noticed the append (IsA<Widget>) and add_suffix (IsA gtk::Widget ) require different inputs, but i cannot make any sense of it.
I thought a gtk::Button was a gtk::Widget ?

In reply to @aaron:matrix.aaron-erhardt.de
This sounds like you are using a version of libadwaita that's not compatible with the gtk-rs version. Rust usually shows errors like this (when the exact same type or trait is somehow incompatible) when you have the same crate in two different versions in your deps.