Need help for second floor Back-to-Back Bathrooms

I have to install new drainage for two bathrooms upstairs, which are in a mirror layout against a common wall (tub / toilet / sink) I have the shower at 36 wide", the toilet drain is 16" from the shower curb, and the sink drain about 4ft from the toilet drain. I need help with lay out that will work with the joists that are in place. The existing dump stack to the sewer is 4" and shown at the bottom of the picture. That dump stack will be concealed within in a 2x6 wall, so there is room to work with merging pipes if need be at that wall, it will not be load bearing. The bathrooms upstairs are approximately 10x6

I have absolutely no idea what those lines are supposed to represent, but it appears that your joists may not be deep enough to do any of it, and the framework might block the piping also.

the lines lead to circles that could be the potential cut out locations for where the plumbing fixture drain(s) would be. The bathrooms that where there before rotted out the sub-floor and weakened the old joists. this is on a new 2"x8" joist system with sister boards (2"x4") to level out the ceiling. The cavity is about 9" tall at this location. This is all new sub-flooring and nothing has been roughed in for the bathrooms at this point. The red lines that go up the middle is the wall in-between the bathrooms upstairs.

I am thinking of running the toilet drain tubes (2 of them) parallel to the joists, merging the tub / shower along the way, and then merging the 2 bathrooms at the stack with a wye...? i am going to build an archway wall that encloses the 4" waste stack, that would conceal piping that drops under the joists at that point.

Is that a good description or should I redo the image?

Whats the deal with the 28 or more shims between the floor joists and the subfloor?

Are those 2x6 joists?

2x8 joists... the block wall has a sill that is about 1" higher on that side of the house, so the upstairs floor tilts. There are 2x4" nailer boards attached to the joists, so the downstairs ceiling will be level when the drywall goes up. The total cavity depth of the ceiling is about 9" (7.25" for the joist, plus ~1.5" for the nailer board lowering the ceiling height. That is what the lighter color (newer wood) is. For the shims there is a low spot in the sub-floor, they were used to try and level up the sub-floor as best as possible. Planning on using additional layer of ply on the bathroom floors before finish floor goes down.

Those shims are not really an acceptable way to level a floor. You will likely have movement. I would hesitate to put ceramic down on that floor, no matter how stiff you think it is.

the shims ends can be cut flush and then sister the joists from underneath to strengthen floor if need be, but for the moment trying to get a rough idea for the waste drains at this point... the bathrooms previously were carpeted with outdoor carpet.

There are so many ways to do the plumbing wrong, and so few to do it right, that there is no way I would try to describe how to do it. No matter how precised the description would be, you could still misinterpret it and screw it up. This is not really a DIY job, regardless of how proficient you consider yourself.

If you replaced the floor joist the sub floor should be flush against the joist, not shimmed up and the floor leveled differently. The only thing that look's OK is the wiring. You better get a GC framer to look at your work and a licensed plummer to do the rough in. A bathroom is the second most expensive room in the home and after it is done, it's awfully expensive to undo things and then redo them,