We currently have our lightroom catalg running through dropbox, since we have over 28,000 images we do this to keep it in the cloud so I dont have any memory issues on my local machine. This was working just fine until last week. I have upgraded to a new macbook pro (running Ventura), downloaded the newest version of lightroom classic and now I can not connect to the catalog. It says the file is corrupt and to choose a new catalog. I have ttried to connect to the backup and to older versions of the catalog and I get the same error message every time.
Placing a Lightroom Classic catalog in Dropbox does nothing for the amount of memory used or any other memory issues. Dropbox syncs a local folder, so the catalog will still be a local file in that local folder. Even if you set the file to 'online only' in Dropbox, that still does not change it. As soon as you (read: Lightroom Classic) try to open the file, it is downloaded so it's a local copy again.
Using 'online only' may be the explanation of your problem however. There is a problem with Dropbox and the latest versions of MacOS, where this automatic downloading does not work for anything but the Mac Finder. That may be the reason why Lightroom says the catalog is corrupted and why repairing it fails. Lightroom Classic cannot work with an online catalog. It needs to download the file, but that does not happen because of this issue. So if the file is indeed set to 'online only', then change this to having a local copy. And if your memory theory is the only reason to place the catalog in Dropbox, then consider moving it out of Dropbox completely.
BTW: a catalog with 28,000 images is small. My own catalog is around 200,000 images and there are photographers who have a catalog with one million images or more, without having memory issues.
Adding a general comment ... placing a catalog file in a folder that is synced with cloud storage (DropBox, OneDrive, etc.) will be a potential cause of catalog corruption. It is really not a good idea to do this (as many people have found out over the years).
Nevertheless, DropBox should have history of several recent versions of this catalog file, you need to see if any of those will open once you download it to your hard drive (and place it in a folder that is NOT synced to cloud storage).