To slow down the squat you need to add compression damping - something not available on the OE shock.
The chances are the adjuster adds a little of both, many shocks with a single "rebound" adjuster will actually add both, but with a fixed ratio like 3:1 rebound to compression.
The problem is rebound is to reduce the speed the shock returns, so by adding more the bike will squat and then stay there, rather than squat and return, in a way a pillion is already adding a load of rebound damping.
Before I replaced my OE shock I would run full pre-load for two-up and then add a bit of rebound damping to prevent wallow, similarly theory for solo - run the suspension as soft as possible without wallowing in bends.
The OE shocks soft spring and lack of compression adjustment means you are not going to get a great setup, and bigger "improvements" are likely to come from riding smoother to avoid upsetting the apple cart - or to buy a better shock.
IIRC I found the OE shock with max pre-load and about 5-6 turns out on pre-load was about the best compromise, at a certain point adding more damping to mine just made it feel very harsh so I settled for a reduced wallow and decent ride quality - but it would still wallow a bit when pushed on faster bends and ground clearance was still limited - a Wilburs shock made a huge difference and solved all these problems.