All of the icons on the rounded rectangle devices are rounded rectangles. All of the icons on the watch are circles. This leads me to believe that the Apple Watch was always intended to eventually be round.
My own phone (6), was very noticeably slower and less capable over the 3 years that I used it. So much of this can be chalked up to declining battery performance and the wearing out of NAND flash cells over time. A new 6 running the latest OS would probably be fine.