USB DAC/amps are less impressive on Android Spotify because the standard Android driver is up-sampling. The Dragonfly Red runs at 44.1kHz with Spotify on my PC but on Android it up-samples to 96kHz causing glitching. Other DACs have similar issues. The proposal is for Spotify to access USB DACs directly, bypassing the Android driver.
Audiophile music players already take this approach, e.g. USB Audio Player PRO (UAPP) and Neutron. Tidal works with UAPP.
External USB headphone DAC/amps for Android phones are becoming a thing. What Hi-Fi has a Best DACs page that mainly lists portable audiophile kit. I would prefer to carry a phone and a DAC rather than a phone and a portable music player because it is more fun to have access to the whole Spotify catalogue.