Bathroom plumbing project

I have a house built in 1920 and I have a few questions I need some advice on. The upstairs and downstairs bathrooms mirror each other and are stacked on top of each other. Both bathrooms are currenlty gutted.

The upstairs tub, sink and toilet drain into a 4-inch cast iron stack which vents through the roof. The tub and sink drain lines ran under the upstairs bathroom floor to the cast iron pipe in notches they cut out of the floor joists.

The downstairs tub, sink and toilet tie into the same cast iron stack as the upstairs but in the crawl space.

The 4-inch cast iron stack is currently the only vent for the two bathrooms and I want to fix this.

My plan was to tie the upstairs and downstairs sinks together on a run of 2-inch pvc and the same for both tubs, each with vent lines running into the attic. I have read For Pros by Pros Remodel plumbing and have a decent understanding of how to vent the system but my main question is below:

1. As no previous drain or vent lines have been run in the walls can I cut the holes necessary to run 2-inch pipe through the floor and top plate?

The wall in question is between floor joists but I'm concerned, due to my own inexperience that this could cause structural issue. Am I wrong?

2. Is 2-inch PVC sufficient size to drain two tubs?
3. Can I use AAVs in the attic on both the tub and sink vent lines?

I planned on replacing the portion of the 4-inch cast iron that's been wet for the last 80+ year but leave the upper portion going through the metal roof. So using AAVs would allow me to do this otherwise I need to take out the entire run so I can tie the tub and sink vents into it.

THank you for any advice you can give

If you can adequately support the CI, you can cut it. Some locales don't allow you to have CI above and resting on a PVC section (the weight can be a problem on the plastic), but horizontally should be okay if the sections are hung properly.

To vent the lower bathroom per today's specs, you'd need to tie those vents together and reconnect above the upstairs drains - nominally at least 42" or 6" above the highest flood plane (probably the vanity sink), so it could be either reconnected in the walls or in the attic before it goes out through the roof.

A vent stops being a vent whenever there's a drain above it.