Herzlich willkommen!
You can have a look at the DELE B2 and C1 exams on the Instituto Cervantes site. If you do the models at home, they help you to see more clearly your strong and weak points.
https://examenes.cervantes.es/es/dele/preparar-pruebaThis is a good general article on resources:
https://www.spanishwithvicente.com/aprender-espanol/libros-para-aprender-espanol/In my opinion C de C1 (Difusión) is the best textbook for C1, for B2 Bitácora (Difusión as well). They are aimed for classroom use, but if you have the solutions it's not so hard to work with them. You can find the solutions either in the accompanying teacher's manual or as a registered premium user of their online campus:
https://campus.difusion.comYou can see the samples of the textbooks here:
https://www.difusion.comThere's an option for each textbook to buy it with a year's premium membership, but if you don't insists on having a physical book you can buy only the membership from the campus, and then for 27 Euros you'll have a year's access to all their textbooks in a digital format. Some of them are interactive, so you solve the exercises, and the system corrects them for you. There's also a lot of supplementary material on the platform, I highly recommend it.
I had the membership and the textbooks in print, but I gained my knowledge mostly through native media. It's surely been more pleasant this way, but it was really time-consuming, and I ended up having huge gaps in my grammar knowledge. If I had been able to study thoroughly these materials, I could have saved a lot of time. I think a combination of textbooks and native materials is the best option in most cases, on higher levels less textbook, more native media. I mostly watched Netflix and HBO, but the best platforms for Spanish language TV series are Movistar and Atresplayer. Some series I liked are:
Spain - Paquita Salas, Aquí no hay quien viva, Arde Madrid, El día de mañana, Foody Love, La casa de papel (season 1-2), Gran Hotel, Mar de plástico, Vis a Vis, El Internado, El tiempo entre costuras
LatAm - Epitafios, El jardín de bronce, La casa de las Flores, Oscuro deseo
I've read some popular fiction, but if it's from Spain, it can be very trashy to the point of irritating, so watch out. If you check the logs, you can find a lot of recommendations for novels.
I'm subscribed to the El País online, and I can really recommend their articles as well. You can read something like 10 articles for free each month, so you can see whether you like it before paying them anything.