DIY Master Bathroom retrofit plumbing & vent stack

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Charlotte, NC
Hi All - new to the page and this site is fantastic! Thank you to everyone for you help and guidance.

I'm remodelling my master bathroom on the 2nd floor of my house and when I had demo'd everything came across what looks like a wild setup. There was a shower / pony wall combo and then alcove tub with center drain. Then there is the botch job 3" vent stack that emerges from the top of the 3" main line with a 3" x 3" x 2" wye. That vent stack penetrates through the roof after some twists and turns and has been leaking for years apparently.

Bathroom small.jpg

My problem now is I purchased a left drain alcove tub, will rebuild the pony wall but now the crazy vent stack is in the way of the new tub overflow and drain line. The water lines are also in the way but can be easily moved.
Proposed layout.jpg

Bathroom proposed small .jpg

Since there is not space in the floor joist bay to add the vent stack wye and a 90 turn to the bath drain p-trap I was thinking to do the following:

1. Redirect the vent stack by laying the 3"x3"x2" wye on its side so the vent moves horizontally under the floor to the exterior wall and then transitions vertical to move up through the roof to get it out of the way.

2. Add 2x 90 turns to the new bath drain line, cut a new 2" hole in the floor joist so this drain line can form a U and tie back into the 3"x3"x2" wye that will be on its side.

Also, could I change the 3" vent for a 2" vent pipe. We are in Charlotte, NC.

I'm new to all of this so just trying to figure out the best solution to what seems to be an impossible problem.

Any and all suggestions welcome or theres probably a much better way than what I've though about.

Thanks so much.
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