I know this is possibly the most delayed answer ever, but... finally the game IS coming to Steam!
https://store.steampowered.com/app/1298280/Captain_Disaster_in_Death_Has_A_Milli...
If someone buys / bought it on Itch they should be able to get a Steam key via the Itch page once it's released (29th May 2020) and I've set up the distribution.