I ended up occasionally chugging about half a liter of water before going, because this started happening.

That seems to actually do something very useful when I think I've not had enough water prior, but I don't actually know the details of how the body moves and uses that water. It intuitively feels like 2-3 minutes should be too short of a time-span to start doing useful work, but it still seems to work.

In general not using piddly little 0.2l glasses and instead keeping a large glass or bottle with me isn't just helpful, it's vitally important. I simply can't get by occasionally drinking small glasses of water. Other than the first day I'd resolve to start drinking more water again, I never went to fill them up enough times to actually get sufficient hydration for my size.

Additionally, I've learnt to feel my hydration on my lips, if they're dry I need a drink. But I am an unreliable, irrational actor. I don't always pay enough attention, or stay on top of things even if I notice, so I'm likely to resort to desperately chugging water again.

After holding in a shit for a couple hours yeah. Literally shitting bricks at that point.

It’s like a fuckin’ howitzer when it reaches the release point, for real

A sound like from a gun, followed by the shattering of porcelain.

As someone who tangentially studies IBS, have you tried to follow a low-FODMAP diet? It doesn't remove everything, but makes it easier and is currently the best approach according to studies.