Please post photos

I keep trying but not sure where they go! Will try again.

If you are doing hydronic floor heat, then that will be enough to heat the space. Electric floor heat would not heat the space.

The space looks tight and you will always be limited by the door swing. I wouldn't want to expand the shower towards the sloping ceiling because that always ends up feeling cramped itself. So why not just get rid of all the shower glass and the curb and make the entire room a "wet room". That might be tricky in an older home, but you could apply a waterproof membrane over the entire room. If you went with an overhead rain shower you'd limit the spread of the spray. You could also have a curtain on a curving rod that would limit overspray and when pulled back it would leave the room feeling nice and open.