If the starter relay is bad (not the connections), the starter will spin without engaging into the starter wheel.

True, but the relay might have two functions. One described is about physically moving the gear to engage the flywheel. The other function is to connect the starter to the battery. It could be wimpy at the second but still ok at the first.

Divide and conquer is a great plan for finding this. You already started measuring the + and - paths separately. Maybe look in more detail. Can you measure to the wire in the middle to figure if a particular connection or the relay is most of the drop. Assuming the bolted connections are clean and tight, that leaves the crimps where the wire ends are connected to the lugs, the starter relay internals, and the wire itself where it flexes going up and down. Maybe the voltmeter can isolate further.

Good luck.