This bathroom sink drip leak shouldn't be giving this much trouble, even for an amateur - rubber gasket won't tighten flush

I've been dealing with this drip leak since we moved in. I bought a new drain assembly (chromed plastic) and installed according to instructions, but water still drips from around the rubber gasket, same spot as the old assembly.

Picture here

The gap could come from a few reasons, but I don't know which is the root cause of the issue:

1. The gasket's raised inner circle doesn't quite fit into the sink basin. The old gasket did fit. I could try sanding the basin to make the hole bigger for the new gasket.

2. The gasket is threaded, but has too much friction to rotate. After screwing on the flange, should I push it straight up, stripping the threads? Or put the gasket where it needs to be, and then screw on the flange?

3. The sink basin appears to be misaligned. Push the drain assembly straight up, and it's pointed slightly towards the faucet, with no room to screw on the flange. Maybe I could sand that level, too? If the basin needs to be replaced, I will probably just hire someone.

Which doors should I look behind? Thanks so much for any help.