Hi and welcome! I use 2 Sennheiser open-backs myself -both HD598s. They are both excellent for me, but they probably wouldn't meet your "good bass and treble" expectations, even if you could locate a great used pair. I would consider the beyerdynamic DT990s. Note that there are 80 ohm and 250-ohm models. I don't have any personal experience with either, but I've read and heard that they have good bass, and they should be available to you for well under 200 pounds. I hope you can sample both and return the pair you like less. If it was only your phone, I'd recommend the 80-ohm one, but your Dragonfly may enable a 250-ohm option which you may prefer. I use 2 HiBy DAPs, so I've also only read about the Dragonfly. I only use my Galaxy phone music for convenience in my car and through Bluetooth.
Both DT990 models are available to me currently for $179 at returns-are-so-easy-amazondotcom.
Enjoy your shopping and eventual selection.