That's a tough one.

1. Not that many compact phones right now in general. Sub-6" is a dying breed and anything sub-5.5" is almost gone. Best bet would be some older phones but they'll all struggle with the modern web and android apps.

2. A solid camera narrows your choice down to Ubuntu Touch. Ubuntu Touch uses a compatibility layer for accessing Android drivers and firmware which PostmarketOS strongly discourages, so the absolute majority of pmOS ports have no working camera at all due to a lot of technical difficulties.

So for now look at what supported devices UBPorts has to offer.

Those are for the IC chips' pins, not for Dupont headers. The corresponding female connectors for those are these. Alternatively you'll have to use the IDC pin headers.___