I would recommend a separate Android device. The controls are easier to use & more intuitive. Say, for example, the GPD XD.
The power of the system goes up to Dreamcast level. As you can or have seen from images, it has dual-analog/mapping functions/l2,r2, l3, r3/ Legacy ROM as an option as well.
If you were to just get an Android phone, the controls would always be clunky & the battery would drain fast. I own & have tested numerous times... the GPD XD has the best battery of any Android device. Period. Even on Legacy ROM 1.6 or 1.8, the battery lasts around 8-10 hrs of playtime. Fact.
Check out my site over at dreamesper.com; I list the XD at a very, very affordable price.