here are the upgrade issues:
https://learn.microsoft.com/en-us/dotnet/framework/migration-guide/runtime/4.5.2-4.8
as there are runtime difference, I'd update the nuget packages to highest framework they support. this should be either 4.6 or standard 2.0. I'd alway pick the standard 2.0 version if supported.
if you cleanup all the nuget packages you should be able to remove the BCL libraries and remove from the build. I'd also upgrade any separate projects to the new project file format and netstandard 2.0 if possible. Unfortunately the new format does not support asp.net projects.