It is symmetrical across the diagonal so we will only look at the top left quarter, then multiply by 2 when we are done. Because it is 1/4 of the rectangle it has area 1/4 of the original. 1/4 x 8x5 = 10. This leaves two right triangles and the grey area. The top right triangle has a base of (8/2)-1=3 And height of 5/2=2.5 Area = 1/2 x 3 x 2.5 = 3.75 The bottom left triangle has height of (5/2)-1=1.5 And base of 8/2=4 Area = 1/2 x 1.5 x 4 = 3 Top left grey area is the area of the the quarter minus the areas of the two right triangles = 10 - 3.75 - 3 = 3.25 There are two of these gray areas so final answer is 2 x 3.25 = 6.5
Look into Bangle.js, it’s what I have, it fits some, not all of your requirements.
It’s open source, both the hardware and the software so no lock ins whatsoever.
They advertise a 2 week battery, mine lasts closer to a month between charges.
You have to install any features you want (every feature is opt in, not opt out).
There’s no speaker, microphone, or keyboard though, so no calling or texting straight from the watch. It’s currently £77 before shipping.