Update: Hello all. Recently, I've decided to take my ducklings outside for much longer than I've used to, sometimes even taking them on walks. I've also allowed them to roam in my home more. When my ducklings would bite me too hard, I would tell them "No!" and firmly put my hand on them and lightly pushing them down until their beaks could touch the floor, immobilizing them but still keeping them calm. They quit biting me after that.
Then, I've decided to be very calm and gentle with my ducklings, but I touch and interact with them much more now, and I've allowed the neighbors kids to pet and hold them, as long as they be gentle and careful. I also stopped forcing them in baths, and although they've become very stinky, they are still allowed access to a tub of water which they usually drink out of.
The results were surprising. My ducks don't squeal as much anymore, instead either chattering with each other and me, or silently sleeping. They're now willing to eat out my hand, and sometimes even gently nibble me, the more affectionate type of duck bite, which I was used to. They also don't poop as much inside the house, which was a much welcome benefit. Thank you all for the help! While my ducklings aren't quite as affectionate as was my late young duckling, they've become much more friendly and I'm very joyful of it. I wonder if it's too late to get them to learn to follow me around.