Altium Designer Rules - Nets with different width
Maybe you can add to your net-rule a room. So only the net within this certain room needs to have the bigger width (InNet('') and InRoom('') or similar).

You can also set this in the schematic. I have seen this but i don't know how to do this. Its a rectangle around the nets which specifies the needs for them.

My experience showed that it's easier to set this manually than with such specific rules. If it's just a small pcb I think then it's not worth it dealing with complicated time consuming rule settings.