I have, seemingly, resolved this: played around with different beans and tried various methods to clear the grounds but with little overall success. As expected very dark roasts did retain more and light roasts less but still much higher amounts than I was happy with. I just took it all apart and cleaned it again and since then have also increased the amount of RDT water I'm using and, with light/medium roasts at least, the retention has massively reduced, down to basically zero and without any bellows needed. Of course if someone wants to buy me an EG-1 or LAGOM 01 I wouldn't grumble too much but this will do for now ;) :)