Photo library won't import back on Mojave after Catalina

I can offer you a work around.

  1. Go to you Pictures folder
  2. Right click your Photos library, "Show Package Contents"
  3. All photos are in the "Masters" folder. Create a new Photo library and import.

At 178GB I'm guessing you have some level of organization, folders, etc. I'm unsure if that's recoverable.