Don't worry about the receive buffer. Every time you run the web100 test, it will give you a different suggestion on buffer size. Network traffic determines their suggestion of size.
The Rwin suggested on DslReports starts at a low range for your connection. You would be ok with 10220. I have been tweaking PCs for years and know per your tests and specs that your Rwin can be pushed to 21780 safely(based on an MTU of 1492).
Your connection looks good.
Enjoy!
Forgot to add: If you want to experiment further with rwins, stay between 10220 & 21900.
Try: 13140, 16060, 18980 &