The two Euphorbias in the vicinity are trigona and lactea. The best way to tell them apart is usually to look at the orientation of the branches. E. trigona branches grow upward not outward. They are parallel to the main stem, pressed up against it.
Now maybe in this low light situation the stems are weaker and floppier, and that explains the appearance. I don't know. In any case the care is exactly the same.
Those plants have reached the upper limit of what is possible given the light they are getting through that window. With tall Euphorbias and cacti indoors, the sun stops hitting the top of the plant at some point, and that's when they go south. To be honest I'm kind of amazed at how tall they are in the first place. That must be a bright window.
I can't offer much advice about rescue, I don't think there really is a good long term solution here. Given you are in the northlands, light is always going to be an issue for sun-loving cacti and succulents in the winter. I mean, you could arrange some kind of artificial light and maybe revive the possibilities, but that would be ugly and expensive and worst of all quite annoying to the humans sharing the space (otherwise it's probably not bright enough).
So maybe prune down to half the height, removing whatever branches are not conducive to the new size. And/or restart from a cutting, which can be done fairly reliably but only during the warmer and brighter months. I can advise there.
The important thing is to handle the plant with care when you cut or prune it. Wear gloves, do not touch the sap, don't get it in your eyes. Wash hands afterward. Prepare for a mess afterwards (lay down some cardboard or similar underneath to catch the drippings from cut parts). Once the sap is dry, it isn't nearly as dangerous.
