You don’t say what language or framework you used. If you used TypeScript, problems like userID vs. user_id can’t happen, and you can explicit mark fields as optional, meaning the compiler will complain if you try to use it without checking.

Many of the database problems you mention are not specific to Firebase, but apply in equal measure to any noSQL database. The issues with normalization and denormalization have been well understood for years. Yes, you need knowledgeable designers.