I want it to open in Google Maps instead of Apple Maps and instead of opening Message with the location then just open up the share sheet in iOS and iPadOS.
What actions (e.g. copy) are shown in the Share Sheet, are defined by the developers of the app. In that case the Shortcuts app.
Only your own Share Sheet shortcuts will be listed below.
What is shown in the horizontally scrolling list of apps is defined by the receiving app (in this case Messages) and seems as if Messages does not accept shares from the Shortcuts app. Not sure why, because normally you can share both URL data types, as well as text types (which I also tried) without any problems.
Because of that I used the general https://maps.google.com/?q=… link and not the iOS app specific one comgooglemaps://?q=...
If someone taps the link and the Google Maps app is not installed it will just open the default browser with a new tab for Google Maps pointed to your current location specified in the link.