I think if you're aiming to sound good on phone speakers, which I honestly can't figure out why, but if you are then your best bet is to write to that medium. I would design your music to fit easily in that speaker range. It' s not a bad goal. I don't know about these days but 20 years ago people were making a killing selling ringtones. But, I don't hear about that market any more.
Personally, I would mix your music to sound good on youtube by mixing it in headphones and in earbuds and small computer speakers.
I often wondered why Billy Eilish music sounded so muffled to me then I saw that her brother talked a lot about designing sounds in headphones. Making it sound right on headphones, then it dawned on me, 99% of the population his age only listen to music on headphones and ear buds. Dude is way smarter than I am, because I'm still creating music like it belongs on huge movie speakers.