Need guidance for venting a new rough-in for basement bathroom
The new stack in the top right should connect downstream of the toilet. It can pass behind the toilet and then run parallel to the side wall to the corner. This leaves you with a wet vent via the vanity that servers the bathtub and toilet:

                     Vanity    New Stack
                       |          |
                       |          |
   Shower ------------\|          |
                       |          |
                       |   Toilet |
                       |/----     |
                       |          |
                       |          |
                       |/---------/
                       |
                       |
                       |


As a minimum, the wet vent would need to be 2” as shown on your drawing.