I’m familiar with this and it’s not difficult but it is very inconvenient.
There is a script here that I believe can be slightly modified to do it for you:
Restrict Wrap Around Mode to Selection - #4 by freyalupen
That is a dual purpose script and the toggle wraparound mode part needs to be deleted.
Then it will either resize to the layer or resize to the selection if there is one.
It could then be separated into a resize to layer script (the most useful one) and a resize to selection script, if you want that too.
The scripts could be assigned in Tools → Scripts → Ten Scripts then triggered by keyboard shortcuts that are listed in Settings → Configure Krita → Keyboard Shortcuts: Action Scripts → Ten Scripts
In use, you’d make a filled rectangle on a layer for each of the different layer sizes/positions that you want. They could be placed at the bottom of the layers docker, out of the way. Then to resize the canvas to your requirements, you’d select the appropriate rectangle-filled layer then trigger the ‘Resize to Layer’ script.
If you need any help in doing that then please create a new topic for it with a category of Develop: Developer Questions.