I think @Rockwell75 gave a great answer already. The only thing I'd add personally is that I'm a pretty big fan of EE too and I also think that the Solaris 2020 has a pretty unique sound in that I feel it's quite dark, musical, technically strong, but not striving for airy trebles, and putting tone and musicality first vs air/spaciousness and the 'reference' ideal.
Having said that, more than many other IEMs I've tried, I've found that the sound signature of the Solaris 2020 can also vary a lot, depending on tips, cables, sources and so on. I tried it with an Eletech Plato cable and the sound was surprisingly different; much more airy and open, more clinical and detailed, but a touch less of that musicality and dark tone that I also love. Also, a big difference listening to it on my iBasso DX220MAX vs my Sony WM1Z DAPs.
When I think of the EE line-up, the one IEM that reminds me most of the Solaris 2020 would be the EE Phantom.
It's a tone/timbre monster, it's exceptionally dark, musical, rich and engaging but still with excellent technical performance and a still decent amount of separation and soundstage size.
I think tonally, it's the most analogous match to the Solaris 2020 from the current EE line up.
I'd probably throw Legend X (original version) into the ring as well, as it again is a bit more of a dark and rich tuning, as well as an outstanding technical performance.
And, unlike the all-BA Phantom, it's a DD-hybrid IEM, so there's more commonalities there again
