I think an earlier thread got the ball rolling on flat bars. I give
@Ron Bez for bringing this topic up.
https://electricbikereview.com/forums/threads/my-new-2016-turbo-base-model.2895/#post-36019
I tried an extension but it wasn't adjustable. I think the key may be that your hands and wrists end up in a neutral position.
The distance from the seat to the handlebars (or grips) must determine how far you lean forward. My wrists and thumbs got so bad with a flat bar I built the cruiser in my member photo (left). This works, but it seems to put a lot of stress on the rider's back, it's too upright. I bought a third bike a while after building the cruiser and it came with the standard flat bar. I was in pain the first time I rode it. I got rid of it immediately.
This is the bar I ended up with, and the nice grips. I have to be careful from here. I don't want it to start looking cluttered. You do get a lot of room with some of these curved bars.
Wearing gloves and moving your hands around, avoiding the 'death grip', all help.