Bulletdodger27 Not to throw a wrench into the works here, but what you described as corruption with sims not showing up at venues isn't corruption imo. Like Pick-a-boo_doo said, it's probably a mod issue. Corruption is when you game starts throwing a lot of LEs, lots become inaccessible or if you can get into them, they have gone missing, sims disappearing for no reason and other things that stop you from playing the game. I did a search here and found this post: Sims not going to any public venues ( gym, outside public spaces, etc) | EA Forums - 12787059 and there is a bug report for the issue here: Community lots are completely empty | EA Forums - 12682224
Take a look at these two and see if either matches what you have going on.
If not, then the first thing you should do is delete the localthumbcache file and run a repair. If the problem is still present, then test your mods after making sure they are all up to date. While you are doing that, if the mod has settings, then check those too to make sure a setting isn't causing the issue. And if you got your mods from Curseforge, check the creator's blog, website, patreon, etc. to make sure you've got the latest. Curseforge isn't always the most current a lot of times. You don't need to test your CC as CC doesn't usually create this kind of problem. Test your mods one at the time. You can test your CC after the mods if you want to.
There is one thing you can try if sims are still not showing up to venues after you've tested your mods and have done a repair is to increase the number of sims in a zone via an MCCC option. But before you attempt that, you need to make sure that they are not showing up rather than being slow to show up. They could be showing up but are spawning a long way away from the venue. Myshuno Meadows in San Myshuno is a good example of that in my game where the NPCs spawn at the furthest spawn spot rather than the closest one in the park. If that is happening to you, then you just need to wait a few minutes for them to show up at your venue.
As for getting the University diploma back when you move a sim between saves via your Library, you can leave them on the wall, save the house to your Library, reinstall the house in a new save and the diploma along with every other visible collectible will still be there in the new save. But anything that was connected to a sim will no longer be connected. It doesn't matter in most cases. The diploma is a bit different as there is a cheat to get them back and they will be connected. So, once you have the sim in a new save, turn on testingcheats. Then shift click on the sim and look for the pack cheats and find the DU cheat menu and use the option to generate a new diploma. As soon as you do, it'll show up in the sim's inventory.
Hope this helps.