I found a way to split the difference between "Gideon sacrifices herself for Harrow in every universe" and "perspective switch in the second book because previous MC died" in the Harrow Nova AU.
At the end of the first book, Harrow would force Gideon's hand to absorb her soul, but it would be less about self-sacrifice than "this is the only way I'll ever reclaim my birthright as a necromancer." Same outcome, different motive.
However, in the second book, she wouldn't be dead. Only mostly dead. Her soul isn't in her body, but her body is still functioning fine. Better than fine, even. Her death-wounds healed themselves, and, despite being in a vegetative state, her muscles haven't atrophied one bit.
This is because Gideon is hemorrhaging thalergy, just like Anastasia was before they euthanized Samael. Gideon has made Jod promise not to do that and to explore other options, but in the meantime, she's slowly dying just to keep Harrow's corpse breathing. Gideon is still sacrificing herself, but by inches.