For more information on laxatives take a look at https://pastebin.com/SrDtPinN
That article used to be on The Institute which I don't believe is online any longer. Couldn't find it on wayback machine either. but wasn't sure of the domain name they used.
Magnesium Hydroxide, tablets or Milk of Magnesia along with most other forms of Magneisum are Hyper-osmotic laxatives.
Which means they cause a great deal of liquid to enter into the bowel, which basically just gives you diarrhea.
If there isn't a lot of poop to start with, you'll have maybe a big build up .. explode and then a few runny shits. That's it. Depending on how that effects you might have to be careful of sharts for 12 to 24 hours.
Fibre means there is more poop to happen.
If you significantly increae the fibre in your diet you will poop more in terms of frequency and amount. So my recommendation there is powdered Psyllium Husk.
The psyllium forms a bulk that absords a lot of liquid. So magnesium on top of that doesn't have quite the same effect.
my advice is try take 1 teaspoon a day for a few days so your body gets used to it
Then take a teaspoon with breakfast and dinner.
Then when you want a big poop.
Take 2x teaspoons every 3 hours with an extra (pint) of water.
At some point you'll find you need to poop more urgently than normal due to the extra fibre.
You'll discover that amount of fibre makes you go say every 6 hours.
At that point take 4x senna tablets or 4x dulcolax tablets.
When they kick in they will empty you out. There will be lots of poop.
To make lots of poop the problem is that this still becomes fairly predictable as you fibre load.
So eat a big meal or take one big dose of fibre and then a couple hours later take 4x dulcolax tablets. That's about a 6 hour alarm clock.