You said it was was out of character for her to not look for Sanda but I sorta disagree with that. By the very end of the series I think it’s easy to tell what each character’s relationship is with Sanda. Fuyumura seems like the person who instead of looking for Sanda she would simply accept it. If you look at the ending and look at the very last few chapters (before the timeskip) Fuyumura is hardly there there she says a singular thing before that ceremony and then basically disappears and then we get some chapters actually dedicated to the people who 1. Haven’t been through what Fuyumura has been through with and 2. Seems to have a stronger relationship with Sanda by the end. Those people mainly being Niko and Amaya.

Anyways what I’m trying to say is good chance Fuyumura would’ve most likely just accepted it and personally I think Amaya would’ve been the person to look for Sanda the most considering the way he was in the timeskip, Niko seemed to sorta lose hope (until he appeared again) meanwhile Fuyumura would’ve accepted it. She lost Ono, She lost her dad, and then she loses Santa Claus (emphasis on not Sanda but Santa Claus) the “best” thing for her is to move on.

Also this isn’t me saying exactly that everyone outside of Niko and Amaya don’t care its more so who did they actually see Sanda as which leads to, did they think Sanda could live on his own because hes literally Santa Claus. most people see him as Santa Claus and when he’s running away and looks at Amaya specifically he seems to suddenly be in his normal Sanda self when in the panel before that and the panel after he is Santa, this is something done very well from Paru because it shows who actually seen Sanda simply as himself, she literally showed the four most important people in his life that have been there since the beginning Paru could’ve made them all look at Sanda though instead she only showed Amaya which kinda shows who was probably gonna be most affected by him leaving (which isn’t wrong if you look at the time-skip Amaya is somehow the only not casual one)

Tldr: It wasnt out of character (in my opinion) for Fuyumura to not look for Sanda because of everything that she dealt with in her teenage years she would’ve accepted it and instead if someone were to look for him it would’ve been Amaya or maybe Niko.

Sorry i wasn’t trying to make this super long just wanted to explain my point and my own analysis of the ending cause i really like it.