The Sketch Shortcut reference is a really useful starting point. At the bottom it shows you how to 'Create Custom Shortcuts'. I tend to add the following manually:

Collapse all groups

Alt + Cmd + C

Sort artboards alphabetically

Cntrl + Shift + P

Reverse sort artboards alphabetically

Cntrl + Shift + O

Swap the position of two objects

• Install Sketch Mate​

• Plugins > Sort > Reverse Positions