This summary/conclusion will not be updated anymore. I have switched to .NET 5 completely, which runs on Windows 7 SP1 and newer.
For class libraries, I will use the targets .NET Standard 1.1, 2.0 and 2.1 at the same time, together with .NET Framework 2.0 and 4.0. Applications on the other hand will always targeted to .NET 5.0 only.
Recommended minimum system requirements