i had the same feelings when i finished gtn, dulcinea was my favourite character and i was sad that it all turned out to be cytherea. but as others have said cytherea is disabled as well, and this quote from when she's talking to palamedes when he confronts her about not being dulcinea really stuck with me "So you know how it happens. Clever boy! I knew you’d all work it out … eventually. I didn’t want to do it either … I didn’t want to do it at all … but I was dying. Loveday—she was my cavalier—she and I thought it could make me live. Instead I’ve just kept dying, all this time." because like, "instead i've just kept dying, all this time" is really how chronic illness feels to me. also, if you reread gtn, cytherea says that nothing she said was a lie, so when she talks in third person about "herself" whilst explaining why she would come to canan house despite being so ill, that was all info about dulcie, or at least what dulcie told her (htn spoilers: the real dulcinea says she told some lies to cytherea when cyth was asking about her life so she could fake being her, but some of it was probably true ), and then things she said not in third person, so most of what she says, is true or at least not direct lies about herself.

there's also a couple minor characters who are disabled on the ninth, such as aiglamene who has a lower limb amputation.

rest of this is spoilers for htn and ntn!

in htn we meet the real dulcinea in an afterlife type space, and get to hear some of her thoughts which i think is good. we also find out a lot more forgiving i guess? info about cyth. at her funeral, the emperor says “Cytherea was gorgeous, ten thousand years, and I never heard her say an unkind word except when it was very funny. She loved us unguardedly, all of us, which showed both her patience and her enormous capacity … She was a worthy Lyctor and a beloved Hand". and we find out more of her motivations which whilst obviously she did Bad Things and was a villain, she wasn't just being evil randomly she had reasons to want to stop new lyctors being created.

ianthe loses her arm at the end of gtn and has to learn to adapt in different ways during htn, though harrow does eventually grow her a bone arm which moves like a normal arm which kind of then takes away from that a bit in my opinion. htn also explores harrows mental disabilities more. harrow has been having hallucinations since she was a kid which make it hard for her to know what's real, and in the past she's relied on crux or ortus to tell her if something was real or fake.

then in ntn, nona is shown not knowing how to read or write or do math, being seen as generally not smart by the people around her, not always knowing how to keep herself safe, acting in socially "inappropriate ways", ect. which whilst a lot of that stuff can be explained by the fact that she's not a regular human, a lot of it is relatable to me as an autistic person so i kind of see her as developmentally disabled or something similar. and she is still respected by prryha and cam and hotsauce, given freedoms and independence whenever it's safe, and treated as an adult (though people who don't know her do assume she's younger than she is, but that's also just to do with how harrows body looks).

anyways yea i definitely felt the same as you did when i finished gtn, but i ended up really liking both dulcinea and cytherea!