what USB or Thunderbolt device would you recommend to back up my photos?

Pretty much any. Speed not that important for backup. I just use a USB3 spinning drive. It happens to be a Western Digital mybook - but that was just because it was on offer at the time.

Only thing I would say - go for a backup drive at least 3x the size of stuff you want to backup, and use time machine to do the backups. Automated and simple.