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@Cwiczuk  Sorry for the late-ish reply.  One other experiment worth doing is trying to load just the .nhd file from Sunset Valley.  Create a new save in SV, save your progress, and quit.  Then go into the folder for the new save and delete the .nhd file.  Copy over the Sunset Valley .nhd file from your affected save, but nothing else, and try to load the new save.  If this works, it's at least a way to recover your sims.

To be clear, I wouldn't recommend playing forward this way; it's more of a proof of concept combined with a plan for rescuing your sims if all else fails.  Whether it works or not, we'll know the next step to take.

The other option is for you to upload this save somewhere, for example Google Drive, and I can take a look.  You can post the link here or send it to me via private message.  If you're interested, please also list the expansions, stuff packs, and store worlds (if any) you currently have installed.  I own everything except Katy Perry, so I can match your content and try to fix the save myself.  I've been able to get through some travel transitions that others haven't, although it's kind of hit or miss.