What is the best way to turn this old bath tub drain into a shower drain?

[Mini Me]

Here is the manual for the shower base I am installing

in the past I have used the 4 1/8" Oatley brass/steel shower drain but considering that the drain that is there sits in a notch in the joist below it I do not think this is an option (see the second picture)
What options do I have here ?
Today all the shower drains are 2". My copper is 1.5" and there is no way I am changing that.
Should I go 2" to the copper and use a Fernco 2" to 1.5" flex coupling or I should use an adapter just below the drain and go 1.5" all the way (including a Fernco flex coupling 1.5" to 1.5" -abs to copper) ?

how reliable are the solvent weld ABS drains ?

What other options do I have ?

Here are a few pictures showing the situation

Here is the context, the position of this drain in the bathroom, this is an upstairs bathroom
the drywall you see under the existing bathroom is the ceiling of the basement apartment living (luckily I will be able to access this from there if anything goes wrong here)

PS: please excuse the manners of our mice

 

[Mini Me]

duplicate

 

[Sponsor]

 

[Breplum]

The 90 is not a drainage fitting.
I wouldn't be involved with improperly reducing what must be a 2" drain into an existing 1.5".

 

[Mini Me]

Well what is already there worked for a bath tub
Any reason any physics principles tha were broken because it was not 2"?
What worked back then in the 60s and it is not working now if we keep this 1.5"?

 

[Mini Me]

I am wondering if it would be possible to use a cap to seal the end where the bath tub overflow was connected and then connect from the drain to the brass (1.5 OD) with something like this
https://www.amazon.ca/Shower-Drain-Perforated-Compatible-Replacement/dp/B0CGTZ6717

 

[GReynolds929]

Tubs are allowed to be 1-1/2", some areas allow showers to be 1-1/2" if it's impractical to install proper 2" pipe. The drain you show in post #5 could possibly work depending how deep it is, I would not cut the existing joist any more without reinforcing it. The existing trap needs cut and routed over next to the joist, then offset the shower drain into the trap. Use a solvent weld bushing in the shower drain and a shielded coupling to adapt from the copper to PVC.

 

[Mini Me]

I was hoping there is a way to connect that side shower drain to the existing brass, and cap the opening where the overflow was.

If this is not possible , I would rather use the offset drain type that discharges vertically, connect the ptrap to it directly and then use shielded flex coupling

Ideally I would like not to have to cut the existing p-trap