Photo Transfer

I just meant that the long-term contributors here don't have as much fresh experience in doing the transition as one might wish. Each different version of Photos has the potential for changing the process, so our information may be stale.

If you used Google Drive to transfer the Photos Library to the other Mac it most likely the library has been damaged since online storage facilities like Google Drive do not that the formatting that is compatible with the complex ecosystem of permissions and hard links in a Photo Library.

This user tip explains where it's safe to store a library: Where is it safe to store a Photos Library - 2019 Version

If you still have the MacBook connect it to the new Mac Mini via Target Disk Mode and copy the library to the Mac Mini.

Unless you have previously downloaded the upgrader tool, it won't be in your utility folder, You can read about it and download it here: Using the iPhoto Library Upgrader tool - Apple Support

If the upgrader cannot fix your library, it might have been damaged by its stay in Google Drive. I don't recall hearing that iPhoto used Apple-specific tricks in its libraries, but Photos does, and Photos libraries can be damaged by trips through syncing sites not designed to handle Photos libraries.

People do do this all the time. Many of us use external drives or direct LAN connections to do the copying (if not iCloud Photos). Also, most of us made the iPhoto-Photos transition long ago.

If you hold the option key while opening photos, it gives you the choice of other libraries to open. If you then choose an Iphoto library it will import that library into photos.

Can take some time if your iphotos library is large.

If you are saying you've tried that and it is not working, then perhaps youve damaged your Iphotos libray in the transfer process. Can you describe exactly how the transfer was done (Is your google drive big enough to copy the whole Iphoto library in one step for example?)