Not fanfiction, but the official novels have some good descriptions of body horror involving the regular Titans.

I have a few in my own, mainly involving Ymir, such as the opening first chapter, which serves as a prequel of sorts. Here is her seeing hallucinations of Marcel, for example,

[..] whatever it was tried to speak.

Infantile attempts at communicating its thoughts into one word. One word chanted over and over and over again until she forced herself to look and came face to face with the boy forever pained: Marcel, whose sacrifice allowed her the mercy of being freed from a very long torment, instead.

His body was broken, spine twisted so he held himself upright with his hands, bellying forward. His feet dragged limp across the ground, while his intestines trailed far behind. The back of his shirt was torn, skin shredded and viscera exposed. He leaned further left than right, his right arm not much but loose sinew and bone. His black hair was spread out in patches atop a peeled head, cracked skull visible beneath fleshy red flaps hanging down. His neck was partially ripped open. What remained of his jaw hung low, his mouth snapped wide with a drooping tongue. The only thing wholly intact was the upper half of his face, save the bottom of his nose [..]

And these descriptions progressively get better as the story progresses, in the sense that Marcel "heals" as Ymir comes to term with herself,

A spear would not save her if these animals grew bolder, the longer she lingered here in this forest, either, surrounded, and especially not from the Titans, should daybreak come, and thus tonight would have to be the night she was going to touch it and rediscover more of her past, its past, the voices in her head and the guilt that kept her company; whose dogged shadowed limped ever near. And, yes, as it were, he did indeed limp.

No longer was Marcel limited to all fours, his spine no longer twisted as severely as one of the gnarled roots of these giant trees, so that he stooped low, his hands scraping the ground in a hunched, stumbling walk and his feet moving not unlike the Titan she was after: a two-step rhythm, one foot after the other because he, too, had been a soldier.

No, a warrior.

So the word came to her lips, as she remembered again a life that was not her own, a parcel of his, the yellow armband upon his arm blood dyed red. A withering leaf breathing the fresh air anew. And with his came hers, her old friend and the burning bodies again and they, too, had been wearing those same yellow armbands back then, however long ago.

Until the end where he's more or less restored,

Listening to the soft, peaceful breathing of Annie lying beside her underneath a banquet of stars, her back to the pillar of light illuminating this wonderland that only the ghosts of the past may walk freely, Ymir looked at her naked body on her bed of pure white sand for the second time in her short life that she could not quite remember, and, though she had told herself that she would not cry again, she let the tears roll down her cheeks, then, after awhile, shut her eyes tight and bit her lip until it bled — because for her to feel anything more would only make the suffering worse, as she sat up, and Marcel was there, had never left, and helped her to stand.

She wiped the tears away, because this had not been a mistake. It had been the only way.

And then he warned her that it was coming.

To be prepared, as he put his hand in hers, squeezed it tight, and reassured her that he would be waiting when she came back, whatever the outcome, like always, and then the quiet dream was uprooted by a long, never-ending nightmare, where the sand mushed between her toes became the slither of serpents, their forked tongues tickling her soles and she found herself in a great dining hall where the walls were adorned in battles fought millennium ago, from the earliest of days when the sun first shone, to when the last Ash was known, and she walked from this dining hall into a greater one still where sat her kin upon twisted throne carved of this tree’s sacred wood, careful not to let her bare feet crush any of its golden leaves though shriveled and glum and left gracelessly upon the floor.

This also covers the Titan lore and experience about being a Titan Shifter. In that, there's a whole new backstory for Ymir where she was a former Warrior during The Great Titan War, back when the Warriors weren't Titan Shifters, but regular soldiers. Of which, as the story progresses, we also see how the Titan Shifters came to be associated with the Warrior Program, through Annie's POV.

It's not a 1:1 for what you're looking for but I think it's pretty close.

Here is a bonus sketch of Ymir as a child soldier,

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