Welcome to the forum. The previous suggestions are all good ones. I have no prep routine other than wet my face and lather so no suggestions there.
I find if my angle and pressure are wrong I get irritation but not to the extent you seem to get. You could try placing the head of the DE flat on your face and rotate the handle downward till you feel the blade. That should give you a starting point for an angle. You have to keep adjusting to maintain that angle as you follow the contours of your face. The pressure on your face should be no more than the weight of the razor with you simply guiding it. Do not try to remove all the stubble in one pass.
Do not use alum block on razor burn as bad as yours is as it would be an unpleasant experience. If the Aqua Velva stings the alum would be even worse.
I have found Nivea for sensitive skin with aloe after shave balm very good for soothing and healing.
I shave daily and like my cold water shaves. Eliminating one thing at a time from your routine is a very good way to try and see what if anything is wrong with your routine. Good luck on finding the cure.
Bob