Where the bathtub meets the surround, there's a ~1cm tiling lip (?).
What should fill this gap? A strip of greenboard or something?

Also, the toilet is currently drained by 4" cast iron (original, house
was built ~1957, Toronto, ON, Canada). I'm going to swtich to ABS
with a low-flush toilet. Is it preferable (for flow-rates, etc.) to
use 3" ABS? It's about a 12' straight run to the basement floor.

TIA (again),
Darryl.