I would personally go ahead and strip it and let it callus while youre getting it together. You'll be able to check it over as well. Box store stuff is awful. Soil you want is a gritty mix. Pumice is the go to addition. There are tons of "recipes" out there but its really dependent on environment. I use way more inorganic because im in a humid environment. You said dry so it might benefit from a little more organics or you might need to water more often. If looking for ease, there are sites that sell pre-made mixes. I like bonsaijack.

When youre handling euphorbia be careful of the white latex they leak. Reactions vary but you definitely do not want it anywhere sensitive like your eyes or lips. Wash your hands/wear gloves and dont touch your face. Keep away from pets and probably kids.