nice list. It is indeed quite rare for this trope in BL. Some listed here I haven't read yet (yay!) I've read quite a number of novels and I know a couple that in some way fits your theme. 

https://www.novelupdates.info/series/the-disabled-tyrants-pet-palm-fish-transmigration/ - Transmigrated into a Carp. Not in a half-fish state but he's either a fish or a man but still goes back and forth either form throughout the story. ML later discovers his identity. 

https://www.novelupdates.info/series/the-tyrants-beloved-empress/ - the merfolk part will only be discovered at later part. It's part of the MC's mysterious origin arc. Story more well known for being a male empress story but  IMO his real identity as a sea creature/semi-god is more interesting. Their forms are basically mermaid but  without the weakness of a fish (cannot live without water)