In general, 3rd-party contacts apps that have access to the Contacts permission to use the Contacts Provider can be an alternative to Google Contacts (or any other stock contacts apps).
The Contacts Provider is the standard approach to share and use contact info between apps.
The Contacts Provider is a powerful and flexible Android component that manages the device's central repository of data about people. The Contacts Provider is the source of data you see in the device's contacts application, and you can also access its data in your own application and transfer data between the device and online services.
The Contacts Provider will interact with the system app Contacts Storage, which is independent of any contacts apps. So, even though the user uninstalls/disables all their contacts apps, the contact info is still stored on the Contacts Storage and can be read by other apps like phone apps, messaging apps, email apps, etc.
So, any famous & reliable 3rd-party contacts apps that request access to the Contacts permission should be feasible as an alternative to Google Contacts.
In the case of Simple Contacts app, it actually has the Contacts permission, and any contacts that are created from this app to a registered account on Contacts Provider can also be accessed by other apps.
On the other hand, the app also allows the contact to be saved on "Phone storage (not visible by other apps)". Based on its source code, this is actually another way to store contacts by using their own local database, which other apps do not know about its existence and do not have permission to access it, thus the contacts cannot be read.
In conclusion, the Simple Contacts app is actually a feasible alternative* for Google Contacts.
*Tested by creating 2 contacts; one with the answerer's Google account which then can be read by Google Messages, another with the phone storage which cannot be accessed by Google Messages.