I usually code/add dialogue to 1-3 scenes, then I go in and read through, add directing/overlays/speech bubbles etc, read through again, then move onto the next couple scenes after it’s perfect.
I need to bounce around otherwise I get bored or demotivated. And if I code too many scenes/dialogue for too long, I start to lose sight of where I’m at in my story. So going back and forth often helps me continue to see the story play out.
Honestly, if I knew then I would do it 
I just try to write only when I’m in the mood to do so, and have no distractions.
For the first question, I’m not really sure. I might be a tiny bit faster because I’ve been coding for so long, but I’ve always tried to take it one step at a time.
I’d start by thinking of the scene taking place, then spotting all the characters on the screen in the general place I wanted them to be using the default positions such as back far right. Then I’d shuffle through the spot directing tool and move my characters around to their positions. When I’m satisfied, then I go through and put their coordinates in my script, save, then preview it to make sure I didn’t mix anything up.
I just try to make sure I have no distractions and I focus on one thing at a time.
Seeing the end result and being proud of what I made is worth it for me 
I don’t like using templates, I prefer doing things myself and then undo an customize it to my liking. I even make all of my own tappable games and menus, and then all of the coding for them, as well. But my biggest piece of advice is to just take your time and try planning the scene and maybe even the character layout a piece of paper first. I’ve wasted lots of time before by just going right in, then not liking how the scene was turning out and then had to redo everything lol