I just reread this again after reading the original novel. The ending feels less sudden in the novel since it's hinted much earlier.
The manga also omits several world building details but also clarifies some others.
In the novel, the Mimics are described looking more like the carcasses of dead frogs.
The Mimics are specifically mentioned to be eco-forming weapons sent by an alien civilization from another planet light years away. The reason they were sent was to find new places to exploit due to overpopulation.
There's a reporter character in the novel who tails Rita around and seemingly knows about the loops. He doesn't do much but he gets saved by Rita and Keiji in the final battle.
Shasta is described as native American. There was feather headpiece gag during the scene where she promised to block Rita's lounge. She also collects gachapon that portrays a fictionalized version of Rita and her team.
The bed bunk guy is mentioned to have a girlfriend in the novel. And his relationship with Keiji is soured by the end of the novel because the girlfriend apparently got accidentally injured by Keiji when the base got assaulted. Keiji is just happy they both made it out alive.
The duel between Keiji and Rita was more epic in the novel IMO. It stretched to all the locations in the base, with Keiji having lost his axe at one point, and only ended at the cafeteria where they had their contest. Keiji described him winning only because he learnt how to fight from watching Rita, instead of how Rita learned from watching Mimics.
The novel also ended not with Keiji looking at the sky, but with him actually drinking the three day old moldy coffee.
I also realized there actually was a line in the novel that described the Jackets similar to costume onesies, in the context how it looked when it's not being worn. I'm guessing that's why they decided to make it look so un-mecha in the upcoming anime adaptation.