WCF may be one of your issues. While there is limited WCF client support in core, there is no server support. You should migrate to another technology. Microsoft recommends gRpc, or you can switch to REST (webapi).
you should also check if there are net core, or .netstandard 2.0+ versions for the NuGet packages you use.
the migration approach I often use, is to convert all library projects to .netstandard 2.0 first. This makes them compatible with .net 6, but keeps you to one code base during the conversion. then just the main project needs converting at the end.