https://www.novelupdates.com/series/after-the-disabled-god-of-war-became-my-concubine/ (very very sick) https://www.novelupdates.com/series/its-not-easy-being-a-master/ https://www.novelupdates.com/series/my-husband-is-suffering-from-a-terminal-illness/ https://www.novelupdates.com/series/transmigrated-into-the-violent-boss-little-mermaid/ https://www.novelupdates.com/series/a-scumbag-always-gets-what-he-deserves/
Qingge I have medicine - ML is sickly Forced to marry the enemy Prince Married Thrice to Salted Fish I woke up and and was pregnant - don't know for sure if this counts Sick Beauty Your Kingdom Undying Patient I rely on beauty to stabilize the country The sickly beauty substitue called it quits My darling sick beauty Transmigrating into the Villain’s Sickly Childhood Friend The Cub I’ve Raised is the Crown Prince of the Empire I also know several tragedies with bad endings with sickly MC, but don't know if you like that.
I only know about these and there are some others but they might not be that sick... you can try using the tag 'sickly characters' in the series finder!