I want to have a way to automatically adapt my layout for every canvas size I want. The first thing I tried to do was to snap two rectangles on the top and the bottom of my canvas, but when I change the size of the canvas, the rectangles aren't attached to the border anymore.

Is there a way for these rectangles to automatically stick to their assigned border every time I change the canvas type?

I just downloaded Inkscape 1.3.2 on my Windows.

I´d draw a rectangle with the size and placement to the canvas and set the stroke to zero Opacity then select all 3 - duplicate - now scale the selection to match new canvas sizes. Disable "When scaling objects, scale the stroke width by the same proportions" and set the bounding box to "visual" in Inkscape->Preferences->Tools.