It’s possible to select/choose that the hand tool is the default tool everytime I open a project? (too weird, I know…)
I don’t think so. It might be possible via scripting, but I’m not sure Aseprite offers events that differentiate between opening files vs changing tabs, and I’m not sure it lets you change the currently selected tool.
However, if you hold space, then you’ll enter hand/pan mode regardless of the tool you have selected, which should be a lot simpler than writing a script.