Donald has given you good advice. I would point out the sharp increases in exposure are more stressful for pretty much all plants (including your large spiny cactus) when accompanied by sharp increases in temperature. There is a sort of multiplier effect with serious heat and blazing overhead sun. So baby steps are good in your situation while the plant toughens up in its new environment. As Donald says it may be a bit of a hassle but it's not forever. When in doubt a position in mostly shade (especially during mid day) ought to be relatively safe, for now.
