Bit of an interesting one, I was watching this video (Great channel for space saving ideas btw) and they mentioned they rendered over the existing tiles in the bathroom. Im trying to get my head around how this would be possible given the surface of the tiles originally.
We painted over tiles years ago. I remember the main thing was prep, We washed with sugar soap, sanded with fine sand paper then painted with an etching liquid before the primer and top coats. I would think render would be the same only you would need to seal at the end. It was a laundry we did up it was still good when we moved on. You could always try something like micro cement https://m.youtube.com/watch?v=0yNULLIDw8o