Unless you're doing depth play, I think you, like many others, are overthinking this.

You don't actually need to do much unless you have an especially touchy GI system.

If you're not doing depth play there isn't much to worry about because the play is occurring in your rectum. You're just rinsing that out, which is easy.

Special diets and the like? Not really needed. If you're regular and tend to poop at the same time every day then just schedule your session after that happens.