I like seeing someone comment on history. I think so much in history is facinating in itself, and even if it isn't, it was what really happened. I wish authors wouldn't try to turn the real into a fiction and pass it off as 'real'.
I am not up on Shinsengumi history the way you are, but while they didn't specify the name of his illness, I think the symptoms looked like TB.
I did know he died of TB, but I didn't realize he was likely so young. But all the Senshengumi were relatively young weren't they. For some reason I'm thinking Kondo was in his 30s.
I'm not familiar with any other manga on the Shinsengumi, although there may well be some out there. What I can point you to is an incredible anime - Hakuouki. I'll let you know upfront that it's not completely historical (and I haven't fact-checked what is for its historicity). It mixes in a heavy dose of fantasy but does a superb job blending it into the historical setting. And the artwork... the artwork alone is worth seeing. It is available as a series or as a movie with several OVAs. If you enjoy things Shinsengumi, I think you will love Hakuouki.
Sad.. but almost close to the historical facts ...
Okita in history passes away from TB ...
I am fascinated with the shinsengumi ...the fact they ere loosely a police force ...( I was PD for 18 yrs before retiring )
I think that's one of the many reasons I search historical facts ..plus ..read ( as close as I can ) historical manga
So many interpretations