Photos in Monterey is basically the same app with some feature enhancements. For details, see the photos section here:
macOS Monterey - New Features - Apple (UK)
If you have a full backup, you can re-install an earlier version of MacOS, and then restore all your files from time machine.
However, if you are going this route, you should take a second backup, as while you are re-installing MacOS, your system drive is erased, and then your time machine backup is your only copy of all your files - if that were to fail you would lose everything. You could use (for example) carbon copy cloner to back up your complete system drive to an additional hard drive.
See also
How to get old versions of macOS – Apple Support (UK)