Hi again Ange
The Nissos Kalymnos runs 3 times a week from Kalymnos to Astypalea. (yes, it's still afloat, just never gets south of Kos these days) The once a week ferry from Rhodes that calls at Astypalea also stops at Kalymnos so that's by far your best bet in the mainstream Dodecanese chain. The downside is that the NK leaves at 7 am, so you'd need an overnight on Kalymnos.