First time solo travelling to Spain for 15 days, would love some advice

So I'll be travelling to Barcelona for a conference in May and decided to extend a trip to explore Spain as I've heard wonderful things about it's culture and cuisine. I've also heard that Catalonia and central/South Spain have a completely different culture, so planning to explore both.

My rough itinerary:

• Barcelona – 4 days
• Girona / Costa Brava – 3 days (planning to visit Lloret de Mar and Palafrugell, not sure where to base myself though)
• Valencia – 2 days
• Madrid – 3 days (with a day trip to Toledo or Segovia)
• Seville – 3 days (Would it be recommended to go on a day trip to Malaga or Granda)
• Barcelona – 1 day (before return flight)

This is my first time travelling solo for this long, I would appreciate advice/tips for travel between cities/Internal transport within cities(I'm paranoid about missing trains lmao), any train passes I should be considering, cost saving trips(I'm a 21 year old student), luggage advice, or food considerations(I love trying new food). I am also open to reconsidering the itinerary(But i don't want to miss out on Madrid and Sevilla atleast)

Planning to stay mostly in hostels. Happy to hear any general insights too(things you wish you'd known, some musts in specific cities, things to avoid etc...).