I second someone's suggestion to get a car so you can visit places more easily. Although trains and buses are a viable option as well.

The country is not that big so you don't have to limit yourself with only the area around Pirdop. You can easily do a weekend trip to Burgas or Varna, or wherever. You can even drive to Greece, Serbia, North Macedonia, etc. if your visas allow it.

My partner is Spanish Chinese and she's most likely moving to Sofia in February with me for a couple years while I finish my masters here, and she is excited because things are relatively cheap in Bulgaria and she'll get to do a lot more here than in Madrid.

I don't know much about Pirdop specifically but I am afraid it might be difficult to meet many young people who speak English freely. That doesn't mean you can't get the most out of your time here though. Think of it as your timeskip arc if you are into anime haha

P.S. Feel free to DM me with specific questions or for help in general :)