Mk6 2001?

Are the battery terminal connections clean, bright and tight?
Check the battery voltage whilst cranking, across the battery terminals themselves. Then check again with the negative test meter lead connected to the engine metalwork. That should eliminate any losses in the connection between the battery and chassis, and chassis and engine.
Then it's a case of following the 12v feed from battery, to solenoid, then to starter.
If you are really lucky you might just feel whether any terminals or connections are slightly warm immediately after cranking - got me out of a pickle once.

Ejej
March 2023