There are laxatives and laxatives.
Traditional laxatives like senna work on a purge approach so can induce a lot of cramping. My understanding is that what is typical in MS folk is a loss of muscular effect and hence bowel motility, so those normal motions become less vigorous.
Macrogol (e.g. Movicol, Laxido) works best pre-emptively, a more normal flow keeping things moving by softening and bloating poo, to make it easier to move in a normal rhythm. However it can cause wind and bloating if taken to excess.
What can also happen is faecal impaction. Poo can harden and then be unwilling to move. You still get the urge to go but after straining, you end up passing this sort of runny liquid, leading you to think it is diarrhoea when the problem is the complete opposite. For these situations, try a stool softener containing docusate sodium, such as DulcoEase capsules. Yes, it softens the poo so you can pass it. Another poo softener is lactulose.
More modern laxatives include Bisacodyl to restore gut rhythm and more subtle than Senna.
If you’ve tried swallowing various tablets, powders & liquids, Suppositories can literally use a different approach! If you can’t restore movement by putting in more from the top, try to undermine it from below. I’ve used bisacodyl suppositories to good effect and they have always worked, but… I daren’t be more than a few yards from the loo! When it happens - and it can be a matter of minutes - it can be sudden! Ideally get some applicators too, for shoving them into place, e.g. Dulcoglide.
Beyond that, you’re looking at enemas, some which can be self- or home-administered. They definitely work - and quickly - but are most invasive and not for routine use.
Worth looking at Dulcolax’s website for their range of products and a product advisor page to select the right product: Constipation Relief Treatment | Dulco®
Yes, I’ve had a life of managing my bowels. Time to shut up I think!