I used the multitargetting approach over the last year to convert most of my code to .NET Standard (about 50KLOC). The latest .NET Standard 2.0 and .NET Core 3.1 make it pretty easy. MS' previous attempts at multiplatforming were hit or miss, but they really seem to have finally gotten it right.

Part of what has made it so much easier is the new "SDK Style Project File Format". It makes manually editing the csproj file mostly easier than using the VS editor for the vast majority of tasks. Which, for better or worse, is necessary to get some of the latest features. Got example, you can enable most of the latest C# 8 features for .NET Framework (which officially is stuck at C# 7.2).