Logistically you need to draw the outline around the rack you plan to be using when you are playing 14.1. You make a valid point when stating that a thicker walled rack like a diamond will result in balls being on the line that might sometimes might be outside the line with a thinner walled rack, but how are you going to manage to rack the balls without marking that remaining ball and then replacing it - kind of a pain and not real accurate in my opinion.
If you really want to minimize the size of the outline of the rack, you could find a rack that fits perfectly tight when placed down on the balls, and draw your line around the inside edge of that rack. Then you could use a racking template such as an accu-rack to rack the balls. Problem then is you won’t easily be able to remove the template from the cloth until all balls are cleared off of that template.
Perhaps one day someone will come up with a specialty rack just to be used for straight pool, with very thin but strong walls all the way around including around the three points.
Whichever rack is used to draw the line - the line should be drawn around the OUTSIDE edges of the rack.