Again, I'm giving you the same lame answer: I'm repeating the recommendations from  Microsoft black belt team.
I have not analysed memory consumption by the OS on our servers in detail.
We used the Azure L8s VMs with 64 GB RAM, giving 4 + (64 - 16)  / 8 = 12 GB to the OS and 52 to MSSQL.
I have not tested a setup with just 4 GB for the OS and 60 GB for MSSQL, but I'm guessing that it would work just fine. I don't see this as the root cause for the performance issues that we experienced.