Does it select the 4.5 assemblies rather than the .NET Standard ones, at least?
Yes, it does.
Perhaps it's enough to have an empty dependency list for those TFN's which you do support, e.g. net45-full. When installing in a net462 project, NuGet references the net45-full assemblies, so I expect it will be smart enough to also use the net45-full dependency list.
AFAIK, the best practice is indeed to specify a dependency list for every single TFN supported by the package, including those TFN's which have no dependencies.