Ideas
I was creating my own save file with custom content and mods. I have all the worlds completely rebuilt. I had about 50 rental units in my game, and I only used the cheat to have 8 units on one lot. Most of the builds were unfurnished, and there were no sims in my save yet. But it wasn't until business and hobbies that I had an issue. Once I downloaded that expansion I did what I went to delete all the sims and bulldoze all the lots. But I realized that the lots would reappear in the world map after I deleted them. Or the world map wouldn't show my build, but it would be there after loading into the lot. Eventually, it got to the point no matter what I would do the lots would revert to whatever the last build I had there was no matter how many times I'd save over it. I haven't had any crashes or long loading screens. I was able to, after having these issues, save builds to my gallery and put them in new saves without having any issues.