Basement Shower and Rough-In Drain Pipe

I am finally ready to work on my shower in the basement and ran into an issue. I noticed all the drains are about 2 or 3 inches. The current rough-in pipe is 1.5"

I plan to order a custom made show base since my size is 46.5 x 32 because of space and the to have enough headroom from a sofit that passes through the shower area. Looks like this was meant for a tub. Some of the concrete was removed by previous owner.

What is the best course of action. not trying to go down the right of prepitch and all these other levels just for a basement shower. Just want to drop something in and be done.

can you use a reducer?

They don't let you put 1-1/2 under slab here. When you put your shower in maybe put a 2x1-1/2 bushing in the drain that goes to pipe.

This is the drain stack about 3 feet from the closest pipe.

I am going to bust up the concrete around it to just make sure I cant put a bigger pipe on there

Also I am guessing I have to redo the sink so it has a vent pipe which I will have to find somewhere to hook it into give there are three big drain pipes in the basement

Full view