Hi, as a trans horror enjoyer who likes characters that are fucked up, I’ve been collecting stories for a while now, so I wanted to give some recommendations. Some of them are more than a little fucked up though, so proceed with caution and check out the trigger warnings:
Psycho Nymph Exile by Porpentine Heartscape (& the short story “this permanent and growing theft”) and can be found for $2 on itch.io. There’s body horror, there’s a fucked up world, the main characters are having a very bad time - tw for many things, but especially abuse, sexual assault, sexism, transphobia, transmisogyny, trauma and ptsd. For this permanent and growing theft tw’s are sexual assault accusations and suicide. (Plus her other work seems to be in the same genre, so if you like it, check that out as well out!)
This one is… hornier than the other books on this list, but if you want to read about a former transmed trans guy, who is now forced to detransition by his mom, meeting up with the guy whose life he ruined by doxxing him and “so much coercive hair-pulling that he may as well be in Ratatouille” check out Transfag Home Video by hag fag on itch.io. Tw are given on the itch.io page.
Alice Stoehr writes short stories with trans women and transfem nonbinary MCs and some of them have so many issues and I love her characters so much <3 (if you like Casey Plett, you might like her style as well!). Check her work out on itch.io here. (tw for suicide and abuse playing a role in some stories)
I haven’t read her work yet, but Angela Quinton writes transfem werewolf transformation stuff with body horror, so maybe that’s something that interests you? (link to itch.io)
(If you have the energy I can definitely also recommend to look through the transgender tag on itch.io. Yes, there’s still a lot of romance with 14 year old protagonists (if not in age, then in spirit), but there’s also really cool other stuff on there.)
There’s also May Leitz’s horror books (again, haven’t read them yet, but I desperately want to) Girl Flesh & Fluids that are supposed to be really good. Check out her bandcamp here to buy them. There’s a lot of trigger warnings to watch out for in her work afaik.
I know you said you already read work by Gretchen Felker-Martin, but have you also read Alison Rumfitt, Eve Harms, Poppy Z. Brite, Hailey Piper, Lor Gislason, Caitlin R Kieran or Eric LaRocca? They are all trans horror authors and what I’ve read from them I enjoyed.
I hope you can find somebody whose work you enjoy on here and I wish you a fun time reading if you do :)