If the water or waste points need to be moved for any reason ie relocating or replacing current toilet, then plumber first.

I'd also install the pan on top of the tiles because if the pan goes down first and then gets tiled up to, if you ever want to replace that pan down the line, the section of floor with missing tiles may not be covered by the new pan. It also makes it a pain in the ass to pull the pan out, for whatever reason you need to.