Well that must have been quite a job, and you did it incredibly carefully, judging by the results.
To give you some perspective, a tree Euphorbia like this can grow 3 feet a year in the ground (per stem) or 1-2 feet a year in containers here. We have a very mild climate and those results are outside. It is kind of amazing to me this plant lasted as long as it did in that situation. Clearly it was getting good care, but it seems like the plant was almost surviving despite the the situation, not because of it. If that makes sense. It's a mega tree in nature, really super hefty plant.
To restart from a cutting... wait until spring and make sure you have a spot with good light. It will be much easier to find a spot for a plant that isn't gigantic and vertically challenged, go for that southerly exposure if you can. I have no experience with lights so can't advise there.
Make a cut where the stem is relatively narrow (so the cutting has a cut surface with minimum surface area), maybe along one of the red lines in the picture.
Use a sharp tool, wear gloves. Be prepared for the sap to drip everywhere for a while from both the mother plant and the cutting. Protect the floor underneath (that sap is super sticky, especially when it dries), protect your own skin from contact with the sap, clean your tools immediately afterward to avoid gunking them up.
Leave the cutting somewhere bright with good air flow for a week or two before potting it up. Wait longer if the cut surface is not at a joint (ie. lots of exposed cut surface area).
Then use a pot with holes at the bottom, maybe 8 inches or so. Use soil with good drainage (like decent potting soil with an equal volume of perlite or pumice mixed in). Try not to bury more stem than necessary to ensure stability. Wait at least another week to water.
From here it's important to water with restraint for several weeks. The roots take a while to develop and any water you put into the soil will have to exit by evaporation in the meantime. Maybe water well once every 2-3 weeks at the start? You will most likely see no new growth on top for months, so be patient. At some point this will happen and that is a sign you can water more freely.
Also be aware this plant is going to require much less water during the winter than during the summer, especially if light is limiting during winter. So adjust the watering interval accordingly. Maybe a 2-4 fold difference in how long you wait before watering. Maybe water short of completion during the darkest and coldest days of winter.
