When I was a youngish teen (about 30 years ago) I had a thick, feather filled double size quilt kind of thing. Not the white material you have on duvets now, but a thicker shiny pink material and I didn't know what it had inside at the time. It desperately needed a wash but definitely wouldn't go in the machine.
As you have done, I put it in the bath. Copious amounts of washing powder and loads of water, which it just sucked up, as it was this massive absorbent quilt thing. Gave it a good hand wash, took ages. There was no way I was going to get it out of the bath though, it was so heavy I couldn't even lift it with my brother's help.
Had pulled the plug to drain the water, and the water just kept trickling out of the duvet down the plug hole for ages. Eventually managed to lay bits of it flat in the bath and squeezed down on it to get as much water out as possible, doing it bit by bit, took hours, squeezing, massaging, treading on it like grapes, everything.
Got it light enough to cart downstairs and out the back door, soaking the floor on the way! Got it over the line but it made the line so low most of the quilt was on the ground, was surprised it didn't break the line. I think I propped the quilt up, spread out on dining chairs either side of the line, to raise it off the floor. Took days to get dry, was outside for days then wrapped it around the hot water tank after it had stopped dripping (didn't want to leave out in rain), the filling had gone into clumps and the middle of the clumps were not drying, had to keep shaking it and physically trying to break the clumps up. This was when I found out it was filled with feathers as they kept poking through the covering and I kept pulling them out.
It took so long to dry it went a bit smelly, like a load of washing does if left in the machine wet for a day or two. There was no way I was going to wash it to get rid of the (not too strong) smell as it was not going to dry again and couldn't afford a new duvet/quilt at the time, so sprayed it with things like deodorant/perfume etc and kept hanging it to dry outside to air when it wasn't raining. It was probably highly flammable afterwards but did smell much nicer by the end. (I don't know if Febreeze was around then, but we certainly didn't have any.)
Took well over a week of my life, that quilt. I remember parts of that time vividly, especially the panic when thinking I might have ruined it and mum was a single parent struggling to make ends meet so couldn't replace it. It had been given to us second hand in the first place so we hadn't had to pay for it originally. It never recovered from that washing properly, some of the pockets of feathers were a bit deflated afterwards, but it was a double so still worked well for just me. I am now much more careful when washing duvets, the white kind with washing instructions on them!