Advice for venting fixtures in bathroom remodel

[Joshua Works]

I'm looking for some advice in laying out the DWV in a bathroom remodel I'm just starting. Here is what it looks like right now, with the new fixtures mocked in:

Here are some basic details (I'm sure you'll let me know if I'm missing something important!):

• This is a second-floor bathroom.
• There's a 4" drain in the floor that I can attach any appropriate fitting to.
• There's an existing 4" cast iron vent.
• The new bathroom will have 3 fixtures: a wall-hung Geberit toilet, shower with Kerdi-Line drain, and lavatory.
• The toilet has a 3" drain that can be run laterally through a new 2x6 wall so I don't have perforate any floor joists.
• There's a new 2x6" wall going in along the far wall to accommodate the toilet, toilet drain and shower supply lines. There's a new "2x7" wall going in along the wall on the right (a 2x4 wall for insulation and a 2x3 wall that sits below the window and provides a ledge in the shower). There's a 2x4 wall going in parallel to the far wall that will contain the vent stack and lavatory drain. Basically I'm saying there'll be plenty of new walls to run vents through if necessary.

My main concern is how to properly vent these fixtures, but I would appreciate any overall suggestions. I'm also curious whether the 4" cast iron vent can be downsized to a smaller PVC vent that could fit into a 2x4 framed wall? I would reconnect to the cast iron roof penetration in the attic.

My main question:
The lavatory seems easy enough to vent and drain. If I tie the shower & toilet drains together with a 4x4x2 Wye, how/where should it be vented?
Thanks!