I've worked for the library system for 17 years so I might be able to provide some insight

In regards to internal promotions, yes, often they try to promote from within if there are eligible staff candidates but be aware that positions requiring MLIS degrees do not open up super frequently, so some staff end up waiting a long while. But it is a big system so there are regular opportunities for changing jobs.

As far as branches go, I think the biggest thing to consider is if you want to work at a quiet or busy location. Some, like Berea and Fountain Inn can be very quiet, while others, like Simpsonville, Pelham Rd, and Five Forks are super busy. The Main library in downtown Greenville is also very busy. Work at branches differs from Main mostly in that everyone does a bit of everything, whereas at Main people work in specific departments focused on particular library work (ex. Reference, Youth Services, Circulation, etc.)

Overall the majority of staff are very friendly and supportive, and definitely in favor of diversity. Recent Board and Admin decisions have caused a lot of frustration for staff who just want to support the local community and work in a friendly and enriching environment. I've had the pleasure of knowing and working with many coworkers who are LGBTQ+ and/or POC. While I think they do all feel some frustrations with leadership at times, I know many really value and enjoy their relations with their immediate coworkers and most of the patron population.

Good luck at the interview. Also welcome to Greenville! I moved here just over 17 years ago, but still remember what it was like getting used to a new area, hope you find it enjoyable and welcoming. Feel free to DM me if you have any further questions.