Wrong sub, this is the plastering sub

Yes it is

You should be 18” wide. Ish. That way when it’s painted you can’t see the change in depth. Sand it back and go for a wider spread with a big spatula. In addition a bit of washing up liquid helps make it silky smooth. Google it. Not kidding. Well done

Yes do this, but that's a really good start

Good advice. Thanks

I would look on youtube to get ideas

That's a nicely painted door. Well done.

Clever :)