First-- do you have a backup? You should make a backup before messing around with your Photos Library!
You're using Monterey, now-- is that correct?
What to you mean by "I can't open the app?" You can't open Photos 5 on Monterey? That makes sense. You shouldn't even have Photos 5 in Monterey. A new version of Photos has a new database-- think an excel spreadsheet with more columns. Then a new, more columns data set doesn't make sense to the older database.
Or do you mean that you can't open Photos 7 on Monterey? That would be odd. In either case, what happens when you double click on the Photos 7 app?
What happens when you close Photos and double click on your (backed up) old Photos Library? What I would expect is that Photos would recognize the old version and add the necessary columns so it's readable by the new database-- and then open it so you can see pictures. What actually happens?