Thank you! I like how I'm a philosopher 50% and the other 50% is me drooling over gacha characters. I also like your take as well it makes a lot of sense the more I've thought about it as it explains her motives and the motives of the Babilim people.
In the story Marduk defeated tiamat but now it looks like he never actually did. This explains that line from Marduk in the new years bull event where he talked about being swallowed countless times. Now that we know Tiamat's ability to split and fuse back together from the dragon veins, they were most likely locked in a stalemate since Tiamat could reform any time Marduk tried to split her and Marduk could rebirth over and over. Tiamat probably continued this for a while until she came to the conclusion that her children wanted to branch out or "leave the nest" so she gave Kingu the tablets of destiny to lead her children in her absence and split herself to form a world for her children to move into the future. Except Kingu was slain and Marduk given the title instead.
This is where we hit the problem you mentioned. It would have been easier if had just been that Marduk defeated her and made heaven and earth with what was deemed useful like their belief system dictated. Tiamat performing an act of sacrificial love created a problem since now she was essentially a part of everything and everyone had an intangible part of her attatched to them (love). Acknowledging or reciprocating her act of love meant the people would have to accept Tiamat's whole being rather than just the "useful" part of her which defied the belief system.
This might explain why the other Babilim transient have been so ambiguous about her story. If you notice, they only talk about what she was (manifold dragon) and what she became (heaven and earth) but never detail her actions, motives, or even what happened between her and Marduk. What they were most likely trying to achieve was to alter the story to fit their cut and dry narrative "evil dragon slain by hero" to make it simple and justify their actions. This is probably another precursor concept that other worlds would use to justify removing anything seen as different from the system (like the exiles). Just slap a label as evil on it and have a nice day. Marduk probably believed it too since his memories don't work the others could have just told him he defeated Tiamat, he was a hero, and he continued on believing that to be the truth. Marduk was meant to be a figurehead, put up on a pedestal as a shining example that justified everything.
This is actually really sad now that I'm typing this out. Marduk seems to slip in moments when he talks where he wished he didn't have to fight Tiamat but he's been deluded by others that is was supposed to happen and had no other choice. He's completely unaware of the truth that Tiamat was not a monster but a mother that loved him and he could have just loved her in return and be comforted. Most likely culprit is Anu, (the previous world rep based on bridal event info) Marduk's father.