Fiio M21 & Aful Explorer vs iPhone 15 Pro & AirPods

Hello,
A few weeks ago, I started organizing my music collection, ripping my original CDs to FLAC, and setting up a small Navidrome server. Currently, I listen to the music on my iPhone 15 using AirPods. I know this isn't the optimal setup, so I'm considering buying the FiiO M21 and Aful Explorer IEMs. What do you think, is this a worthwhile upgrade? I listen to a variety of genres, ranging from metal and hip-hop to jazz as well as audiobooks. One more question: Does it make a difference with the headphones whether I choose the 3.5mm or the 4.4mm version?
Many thanks.