Wow 11/2″ vent terminals WHAT State do you live in to allow this kind of garbage plumbing code?
Never have I ever installed a vent terminal less then 4″ to prevent hoar frost and oscillation effects.
I would do the following have the vent piping increased just below roof level to 4″ by a plumber who isn’t too CHEAP to use this larger sized piping.
I hope your plumbing code also has the decency to out law AAV crap or wet venting garbage.
I am appalled at the caliper of non skilled mongrels now in some apprenticeship programs and the poor home buyers are the real victims of this menace.
I wish you all the luck in the world as sewer gasses are not good for your health and a trap seal is the only protection to prevent these dangerous fumes from entering your home. Call a licensed plumber to see what they can do.
Feel free to E mail me with other possibilities available to you
like installing a FRESH AIR INLET or a double vent running trap on the house main.