How do I disable Wi-Fi roaming? (disable autoconnecting to the strongest AP)
It is sort of possible to do but the wording is not clear.

If you open System Preferences > Network and click on the WiFi icon on the left side of the Network Preferences window you will see two checkboxes below the Network Name pop-up menu. On my Mac it looks like this.



For your situation I would select your preferred network from the Network Name pop-up menu, and check both checkboxes so that it will auto-join your preferred network and force your Mac to ask to join other ones.

That is the only way I know to even get close to the settings you need without 3rd party software.