Yup! In fact, I've never even used the original one. I went straight to the Shanling one, so I don't even know if the ONIX adapter works or not!
Right, or dual use rather? I don't like connecting my phone to the Mojo2Poly with a cable. So when I use the Mojo2Poly outdoors, it requires that I use my phone as a hotspot, so that I can AirPlay from it. Whenever I don't feel like doing that, the XM10 comes out to play.
Yeah, I think the weight might be the biggest drawback for me so far? If they made one with a Ti or Mg chassis, built-in 4.4 and maybe more battery life - even at a significant cost increase - I would seriously consider it.

But then it would be thicc.
I can't confirm that, but I also don't use Spotify, which is what he used to demonstrate that. The response is pretty snappy with TIDAL and Qobuz. Apple Music glitches once in a while if there's a lot of art that needs to be loaded. But overall, it seems reasonable to me.
Unfortunately not lol.
I don't like telling people to buy something, even if it's something that I genuinely like a lot myself. For example, I love Macbeth right now. Outside of other IEMs that I'm currently reviewing, it's pretty much all I listen to these days. And even with that being the case, the biggest recommendation I gave for the Macbeth is that people audition it if they can.
That said, if you're reasonably happy with the JM21, I'd probably stick with that for now... with the potential of moving up to the M21 later. Purely from a functional standpoint, I can't see how you'd be happier with the XM10 at 350% more in terms of cost. Plus, as you mentioned, that Walkman case does seem super fun!