Either way, your gut is going to need help acclimating. Start incorporating the helpful supplements and such ASAP, even if you go gradual with the fiber reduction. You have a full week to sort out each change (or as long as you need) before moving on to another reduction.
According to gutsense.org, Vitamin C powder should be buffered so as not to flush all your electrolytes. I don't know how the NOW vitamin C powder would work, I have only used this one. I used it for several months, but am now using magnesium citrate (200-400 mg depending on my needs). The Vit C powder seemed to stop working at some point. To start with, try 1/4 teaspoon. Every 4 hrs, while awake, take another dose. Next day, increase to 1/2 teaspoon at each dose. The goal is to keep bowel movements comfortable & easy to pass. You can increase the dose up over 10 g per day total if you need to. If you take too much, you'll have loose stools, so just back off a little on the total amount of the day until you find a good amount. That may mean making one dose smaller than the others.
With magnesium citrate, it was advised to me to take 200 mg at every meal, and increase if needed. You could do the same thing with magnesium instead of the Vitamin C powder, but I'm not sure how the doses would differ. I know you can take many grams of Vitamin C in a day and it's not dangerous in the least, but I have no idea about magnesium.
What's your diet like? I would recommend making bone broth & chicken soup to have some easily digestible nutrition foods on hand while your gut recovers. Keep in mind this is going to take a while.