Bad custom content doesn't always identify itself, and it doesn't always cause problems right away. The cumulative effect of a lot of content can also prevent the launcher from working correctly. So it often takes some work to pinpoint the issue.
There are two main approaches to a problem like this. One is to sort through the content in your user data folder a bit at a time, adding something and testing to see whether the launcher still works. If it doesn't, you know that whatever you've just added is the problem, or at least part of it. By the way, options.ini and the log files don't need to be added back. Most of the files in the user data folder will be recreated right away, and it's often helpful to force the game to spawn new ones.
If you don't want to disturb your existing installed custom content, what you can do is create a clean Sims 3 folder, then copy (not drag and drop, copy and paste) your content into the clean folder in batches and test. Since you're copying the files, the originals will still be intact, so you can mess with the copies all you want without worrying. Or, if you find that you've copied all the important content (saves, custom content, store stuff, Library entries) into the new folder and new cc still installs fine, you can just go ahead and play in that new folder.
Here's a list of everything you'll find in your user data folder, with explanations of what you'd want to preserve:
https://sims3.crinrict.com/en/2011/01/faq-user-files.html
The other approach is to install your new custom content into a clean folder and then move it to your existing Sims 3 folder afterwards. The launcher still won't work when your existing folder is in place, but you can easily swap to a clean folder when you want to install something. The only trick to this is that you'll need to renumber the newly created .dbc files (bundled custom content) so they don't have the same name as your existing ones. For example, if you have dcdb0.dbc through dcdb4.dbc in your existing Sims 3 folder, when you create a new folder and install content into that folder, the new .dbc file in DCCache will also be labeled dcdb0. Renumber it to dcdb5.dbc before moving it over, and everything should work together. The entries in the Library folder can just be moved over without doing anything special.
How best to approach this depends in part on your personal preferences. If I were in this position, I'd just install everything into a clean user data folder, but then again I don't have a lot of stuff to install. Otherwise, I'd test in batches and get rid of any files that caused problems with the launcher. But if you don't care about the launcher not working normally, it's fine to go the other route. It's not going to affect the game's program files, and as long as you've backed up your saves and anything else critical, there's no harm in experimenting a bit or playing with possibly borked content.
