I have a very, very slow kitchen drain. I have replaced all the pvc piping between the sink and the drain stack in the wall and have rodded the main drain 20' down with a hand crank 1/4" rotary auger and it is clear, I suspect a clogged vent pipe.

Using the dishwasher it will back up into the sink, but then eventually drain down over a 45 minute wait.

I have acute fear of heights and cannot get on the roof but I do have a full attic and see the cast vent pipe above the kitchen and have access to about 3' of it.. Is it OK to take the sawzall and cut an access hole in the cast vent pipe for the flat steel rodding cable and clear the stack that way? Then I'd fab a thin sheet metal type patch over the hole secured with hose clamps?

This is in a Chicago bungalow about 70 years old.