I personally made the unwise choice of cramming five different apps into a single database about 20 years ago. I then had to deal with the repercussions of that. The bad news is, this is really hard. The good news is, modern tools make it easiser, but not easy.
So, first, absolutely get a single source of truth. You're using SQL Server, so there are a bunch of really good tools that will help you maintain your database in source control. One is provided by Microsoft. It's called SQL Server Data Tools. It only works inside Visual Studio. However, it's a solid tool. If you have an MSDN license to run VS, it's complete free. There are also 3rd party tools available for a cost. They're generally much more sophisticated (Microsoft doesn't give SSDT much love). I work for one of the vendors of such tools, Redgate Software.
Once you're on the path of maintaining the code in source control, the easy part is over. Now you have to figure out how to integrate the teams. Here's what we did. Every team got it's own branch of code (we did this in the old TFS, which sucked, you can use some variant of Git which will be much easier, not easy, just easier). This also means that each team had to have it's own copy of the database (which it sounds like what you're going for anyway). In fact, each developer got their own database and then we had an integration testing database for each team. So the team worked on it's own, independent of the other teams.
Next, we created a master integration database. This is where the teams came together. We had a main branch for the code and teams would have to merge their code semi-regularly into this branch and then we could run automated tests to see if any team had broken the code or structures of other teams.
Then, you have to build a release process that ensures that any one team can't overwrite or supersede the others. This means that teams have to regularly merge from the main branch, changes from the other teams.
It requires quite a bit of discipline and some training to get this all set up. However, it's doable. I'd also suggest implementing a flow control software like Jenkins, Octopus, AWS DevOps or Azure DevOps to assist with automating all of it.
I've worked on tons and tons of documentation around these types of processes. Full disclosure, a lot of the info at that link includes our tooling. However, the processes are what you're looking for and you can find good information there. This is a free ebook on these processes I helped to write that's very product agnostic. Hopefully between these resources you get enough to help out.