Hey there, I guess.
Anyway, to cut this short: one of the songs in the show's first soundtrack volume ("cÅGE", to be specific) was performed by Russian singer, Anna Pingina. So I've decided to google her out of pure curiosity and stumbled upon her website. There I found a post about her role in DARLING in the FRANXX. The post itself is pretty old, but the information in it actually isn't. Here's a link, if anyone is looking for a proof (it's in Russian, of course). The last line says that she performed two songs: "Cage" and "Disregards". "Cage" was already released, but we haven't seen a song called "Disregards" yet.
So, that's all I've got. Now we can speculate: ending songs will probably all be performed by XX:me, so "Disregards" is either an opening song or a song included somewhere inside the show. I think the latter is more likely to happen.
upd. Oh, an English version of this post actually exists.