Nuget PackagesThe second big issue with moving to .NET Standard is that any Nuget packages you consume need to have a .NET Standard support. Any Framework-only packages have to be replaced with something equivalent.
Common packages such as Newtonsoft.JSON already do this, so in many cases you won't need to do anything. Other packages might have a .NET Standard version with a different name. Some packages just won't and you'll need to find alternatives - or perhaps lobby the package owner to create a .NET Standard version.
We used iTextSharp 4.x in our application. This was a .NET Framework only package, but there was an iTextSharp 7 release that did support .NET Standard, but was now a chargeable package, and had a number of API changes from the 4.x version which would have meant a lot of rewriting our PDF generation library to work with it.
A search led us to iTextSharp.LGPLv2.Core which was a .NET Standard port of the 4.x version - it was totally compatible with our existing code, and still under a free licence, so we decided to use this.
Not all .NET Framework functionality is present in .NET Standard 2.0 but there are often Microsoft Nuget packages to covert these. An example is System.Drawing - a lot of the Framework version used the Windows GDI libraries so wasn't going to work for a portable framework like .NET Core/Standard.