Self-dilation is safest when it’s used to maintain an opening created “professionally.” Some folks here do it all the time, and can give you better advice than I can about how/when/whether to do it. You do have to be careful and understand the proper angle and depth of insertion, and the diameter your stricture will tolerate. It can be very painful to enlarge the opening appreciably, so anesthesia is commonly used by the pros.
It’s certainly worth preventing anything from getting trapped behind the stricture, and solid stools are not your friend. Liquid stools are probably safest, but they are unpleasant and irritating. More important than Miralax, I suspect, is to stay away from insoluble fiber or anything that might still be solid at the outlet: skins, seeds, spinach, etc. Drink plenty of water. If this gets extreme then dilation is no longer elective and you might need to get it done in spite of the inconvenient timing. Stay safe, and good luck!