I have had cooling problems last year and had several contractor coming in to tell me what they thought was wrong with my system.
Only one was able to show me (with a remote monitor) why my second floor was not cooling.
There are no ducts going from my returns to my unit in the basement! In all rooms i can see the return openings but the video showed no ducts.
So no matter how much air the fan pulls, it is never coming from the second floor.

Now my question: can I somehow add ducts without opening each single wall from the socond floor to the basement?

Could I run one main duct to the attic and split it uo from there to my bedrooms and hallway?

Or would it be better to add an additional, smaller unit in the attic that will only cool the second floor?

Heating is not a problem since hot air rises and I don't want my bedrooms too hot anyway.

Thanks in advance for your advice,