How can i improve the sound of Iphone 11?

Have you thought about iPhone 11 > Apple Lightning to USB > AQ Dragonfly? 
Dragonfly will be able to decode MQA. I personally utilized this combo when walking - dongle and Dragonfly add a bit to the phone, but not too distracting if you bend the dongle and the Dragonfly lays behind the iPhone (in your pocket for example). 

+1 Toro.

I found a step-down trafo (2:1) improved the SQ of my Meizu phone. A bit bulky to carry around though.

"Have you thought about iPhone 11 > Apple Lightning to USB > AQ Dragonfly?"

This is a good possibility. Getting a  (Apple Lightning to USB) is easy enough, and the Dragonfly isnt big, but i never heard it and wonder if it will sound as good as the DX90.

Or if another mini dac would do better. A quick google search for "best portable dacs" revealed different reviewer opionions. No one agrees which is best - Cobalt, Audiolab M-DAC nano,  Clarus CODA USB DACm,  THX Onyx,  Maktar Spectra X2, etc.