Got it working!
Probably because I am using the Community Edition and don’t have a UiPath Folder under either of the Program Files folders, to get my external DLL to show as an Activity in UiPath Studio, I needed to save the Package to my “C:\Users\%user%\AppData\Local\UiPath\app-%UiPathId%\Packages” folder.
Thanks to Andrew.James for providing the final piece of the puzzle re the Execute method override.