Yes, generally over 50mm wrenches go up by 5mm to 80mm. I imagine the really big metric bolts are much more standardized than the SAE equivalents.
If you truly need to fill in gaps from 40mm to 80mm, you just need to buy the SAE wrenches as they have the sizes.
I used to have a Ultrapro deep and SK USA 5/8 shallow. The broaching on the SK was awful and had manual die grinding on the 1/4 drive hole. I sold them with my old socket collection but I kept the 1/8 SK shallow and deep as they are close enough to the Tekton set.
If you were in Canada you can...
Honda's are indeed expensive. While you can get Honda engines on various power equipment, they haven't allowed another brand to use one on a snowblower since they started making them in the 80's.
Typically shared wrench blanks happen on sizes outside of the common wrench sizes. For example Gearwrench 72t ratchet wrenches 13/16 and 7/8, 15/16 and 1", 20mm, 21mm and 22mm, 24mm and 25mm.
Here's a comparison of my Wright wrenches. The Westward is a Taiwan made by Infar. My other open end wrenches are either Gearwrench or Kabo made Mac with a V groove open end.
The 3/8 is smaller than the 10mm and bigger than my 9mm.
My Wright SAE and metric wrench sets were bought a little over two years ago and there are no shared wrench blanks between sizes. I love these wrenches.
There is lots of used SAE Snap On tools for sale near me but the SAE market is dead up here so they sit for months unsold.
While not Koken, my Stahlwille wobble extensions are stamped:
1/4 - 40 nm
3/8 - 80 nm
1/2 - 200 nm
By their nature wobbles aren't as strong as a normal extension.
Received my Stahlwille locking and wobble extensions from Mister Worker today. I can retire my Mastercraft ones other than the 20" as Stahlwille doesn't make long ones except for a 18" 3/8 but the price is more than double.