OH FUCK. To be honest I have an entire playlist for Cetus, so I have a TON of songs for him + his relationship with Deep. I'll try to list off a few of the big ones and the associations I have for them.

Time's Scar (https://www.youtube.com/watch?v=uhXGgVLbz_M&ab_channel=Veeroos) I know you said instead of a general theme, but like... I really have to include the song that inspired so much about him and that I have a complex opening about him in my head with.
Kaibutsu (https://www.youtube.com/watch?v=j4dpZx-CbKQ&ab_channel=AniMelody%E2%80%93AnimeMusic) - OH MORG ANOTHER BEASTARS OP, YOURE SO ORIGINAL yes I am. The lyrics are about being in love with a girl you can't have and being willing to do anything to be with her, fighting against society and your engineered nature to do so. You don't get more Cetus than that.
Pearl Diver (https://www.youtube.com/watch?v=o3Jq35HZuCg&ab_channel=mitskimitski) - A really beautiful and apt song about a hunter diving deeper and deeper to his death. Driven by his unattainable desire while an unknown speaker looks on with pity and love. I think this one speaks for itself.
The Angry River (https://youtu.be/hfzrRdarnEs) - A very very dim and dreary song reflecting on the beastly nature of the hunters and the tragedy of their fates. Applicable to all of them so I think of a compilation of all of their darkest moments. For Cetus specifically I think about him directly after surviving the Great Silence and killing Iberian Inquisitor pursuers on a dark beach.
The Wolf - (https://www.youtube.com/watch?v=Zqz55W5RbQw&ab_channel=FeverRay-Topic) Bad bitch hunting song, the foghorn in the beginning is so ominous and feels like him coming ashore behind a red sun with nothing but an inhuman lust to hunt in his eyes. Also the lyrics referencing poisonous blood, nice.
Even When the Water's Cold (https://www.youtube.com/watch?v=JEPLFpHopCg&ab_channel=%21%21%21%28ChkChkChk%29-Topic) - A more upbeat and light song that I always think about during his happier moments teaching Deepcolor about things like love and opening herself up to new experiences. But there's also a hint of sad wisdom to it in some lines that remind me of his mentality. On the whole "swimming when the water's cold" is a good metaphor for Cetus' persistence and willingness to take the bad with the good.
Nobody (https://www.youtube.com/watch?v=AMptW7Ij7CI&ab_channel=TheTinyMajority) - This one I always think about during Cetus' time in Iberia, sort of wandering around and bedding people in desperation to feel something. The lyrics are pretty spot on for conveying that sense of self-loathing, cowardice, and yearning. I always imagine him intimate with a different faceless person on each repetition of "nobody."
Another Side/Will Motif - (https://www.youtube.com/watch?v=Zo_uNrHKp_I&ab_channel=SatoruK%C5%8Dsaki-Topic) I imagine first playing during the time that Cetus chooses to spare Deep and sort of being a leitmotif for their relationship, so anytime he hugs her and tells her that he loves her while the weight of their impossible situation crashes down around them : )

This is me doing my BEST to limit it and still it's long. I'm cringe! Thanks for this great question!

This is a bit of a specific context, but I think when Crux is just getting adjusted to being his "new" self after waking up from his coma, he has a lot of depressed concern about himself. Who was he before? Why do so many people fear him? And fitting that, I just think of the song Be Honest by Forrest. https://www.youtube.com/watch?v=Zo1WG4FVl2k&list=PLl4nvk5PTgHmejcBrUwcscOp_VG543WYA&index=10

For Valerian, this is a little more sappy but I think when he's gone awhile without seeing Tourniquet and really misses her, the song Being Human fits his mood (https://www.youtube.com/watch?v=RZ8y37jMX4A&list=PLl4nvk5PTgHmj-1T4atrSPKST5z7Xt5RU)

It's not in a literal sense, he has no regrets with loving Tourniquet for exactly who she is, but the way that the song talks about just taking time to go somewhere and live in the moment. He knows he's not gonna be around as long as she will, and he wants to take the time he knows he has to give her those moments to remember fondly.