Looking at your screenshot, you are already multi targeting (Net Framework 4.8 and Net 7).
In this setup, you won’t be able to use DLLs compiled for .net 7
The official recommendation from McNeel is to ship your plugins for 4.8 (compatibility with R7, Rhino Inside, etc.) but test them in Net7 to make sure everything works as expected.
What should you do? Multi target both your plugin and external library, reference the 4.8 DLL in your plugin.