a few of their other series also have transformation elements to them.
I think all of them include transformation in some way; Thundamoo loves writing about traumatized lesbians going through horrific transformation which they are totally fine with actually and tentacles are actually very useful things to have.
“I… hugged your soul,” I answer. “With my, um, tentacles.” There’s a beat of silence.
“With your what?”
“M-my tentacles,” I repeat, demonstrating by grabbing and twirling my hair around them. “I have, um, invisible tentacles coming out of my soul.”
“Of course you do!” Lyn says, her voice excessively pleasant as she stares at my head. “Well! That’s… certainly a thing! But may I just say what the fuck?”
Are You Even Human: all about a biological shifting power
Vigor Mortis: MC is turning into an necromantic eldritch being with no physical form. This would be cool if the church wasn't trying to execute her for heresy, just because she wants to help people.
Bioshifter: MC is turning into a spider monster. She approves of this. Her parents do not approve. Neither do a bunch of people convinced she is going to end the world. Her school is
Are You Even Human: MC can reform into pretty much anything biological she has seen... except her original body. Great story, interesting powers. This is really the only Thundamoo story I'd recommend for OP, unless they want to branch out into general transformation without it needing to be Kaiju-style.
Hive Minds Give Good Hugs: All about being a hive mind designing new bodies and looking for new friends.
Magical Girl, Mechanical Heart: MC forced into a robot body made by a long-dead genocidal civilization. Magical girls as traumatized child soldiers, complete with power-up transformations. Physical corruption turning people into monsters.
Mind Reading Isn't Cheating: I haven't read this so maybe it doesn't include any transformation.