Planning bathroom renovation - drain and venting questions

Hello gurus,

I am planning to renovate my master bath and I need some help with planning the plumbing. Here's my existing bathroom:

The empty room is currently a walk-in closet. The bathroom is located on the second floor above a closet. I had ceiling partially removed in that closet some while ago, so I was able to take a picture of plumbing under the toilet:


Pipe on the right is the bathtub drain and on the left is the sink drain. I believe the only vent is connected to the sink drain but we can't see it on this photo.

And here's what I want to do:

The former walk-in closet is above garage, so if needed I can get access from the bottom. The tub drain can probably be reversed if needed. I haven't bought the tub yet.

So my question is what is the simplest way to connect drains in this configuration and whether I can get away with the single existing vent?

Thanks!

is your single existing vent the watercloset stack? Every fixture needs to be vented. IMO you are going to have to redo the piping under the floor. You can vent the new wc location with the new fixture but you still are going to have to vent the lav and tub. If you join those vents together you can exit the roof with one vent if its the stack vent. I cant draw online but others can and may speak up. you can wye off the watercloset line with a drain for the tub and lav and then vent those lines, or wye off the stack and feed those fixtures with individual vents.

The existing vent is not stack. You can see on the photo that the stack ends under the toilet. This stack is only used for this bathroom. The vent is connected under the sink to the sink's drain and I believe it must be connected to the main stack vent in the attic.
I realize that will have to change much of the piping, I am just trying to figure out the simplest way. I guess if I switch the toilet with the bidet I could use the existing tub drain for the bidet but what about the rest? I know I am not providing enough information about the underlying construction (I am yet to do some exploratory work), so I can't expect too much but perhaps some ideas...