Thank you for the suggestions.

I hope these are appropriate questions to ask (still trying to learn how to abide by the forum's rules ):

Mortar wise, what do you think about reinforcing it with wire mesh? Any mortar will do or would you recommend some particular mix (not looking for a brand here -- the type),. Structurally strong concrete mixes seem to need 5" to 7" thickness which we cannot do here (I see your point about redoing the whole thing is the right approach).

Would adding a cement board on top of the mortar be a good idea? I am thinking this would reduce how thick the mortar needs to be and maybe it can help avoid crumbling in the long run. Or this is half assing it to the square :)

Thanks!