Shower screed confusion

Hi Everyone, new member here. Have a question and would like your opinions as there seems to be some knowledgeable people here.
My bathroom is on the second level so the floor consists of wooden floor joists, a chipboard floor then
cement board on top and covered in waterproofing.
The old screed looked like sandstone (prob sand/cement) and was REALLY stuck down.
Do I need a bonded or unbonded screed?
Either way the screed can only be 11-12mm thick.
The general consensus I have heard is bonded for concrete ground floor slab and unbonded for upstairs.
Does anyone have experience with this?
What would the best screed mix be for this purpose?
Any needed additives?

at that thickness it needs to be bonded. Use a floor levelling compound. Make sure waterproofing, leveller and tile glue are all compatible ie same company. Be sure to prime carefully

Ok thanks, I thought so as well because it is so thin. The old screed was extremely hard to remove so was probably bonded.
Needed a heavy-duty jack hamer.

Unfortuantely, I've thrown it all out so cannot check if a bonding agent was used.