Will i need to renew this pipe before i install puddle flange

Hello!

Im currently working away at renovating my ensuite (yes, im aware of the risks - i have a year off work and im willing to take the risk to work on some projects)

Its a ground floor bathroom, on a slab. Shower recess steps down. The old shower was waterproofed with a fibreglass type system.
The floor waste pipe is a bend and obviously has some of the waterproofing going down it.
Will my next step here be to cut the slab out around this bend so i can renew it before i install the puddle flange?

And while im asking questions, i will be waterproofing the slab and then screeding over the waterproofing. How smooth should i be preparing the slab for the liquid membrane? it is fairly rough under the old waterproofing, so should i grind this back, do a top up with some floor leveller, or is it ok to leave it rough with a few deep spots?

Thanks very much

Cheers