Patrick Smacchia has a few tips for migrating code from .NET Framework to .NET 5 or even 6:
In January 2020 I wrote the post Not planning now to migrate your .NET 4.8 legacy, is certainly a mistake. Hopefully we followed our own advice and have been migrated most of our non-UI code. This way latest NDepend version 2021.2 can now run analysis, reporting, power tools and API against .NET 5 on Windows, Linux and MacOS.
We learn a few things during this migration journey. Let me expose those in five points:
My most positive experiences with this have come in migrating projects with relatively few third party dependencies. The big problem there is that a fair percentage of older libraries never made the leap to Standard, so you may be stuck with a re-write (or just stuck in general) as a result.