Consider Dr Will Bulsiewicz's advice:
https://www.youtube.com/watch?v=RkjyuEWR1cw
https://theplantfedgut.com/book/
– he's a leading gastroenterologist and one of the ZOE team.
Senna is not the answer. Senna and other laxatives are habit forming and should be unnecessary unless a person lives on donuts and burgers.
Aim to consume at least 30 different plant foods every week as per ZOE recommendations
https://www.youtube.com/watch?v=olKsStenYug
https://zoe.com/learn/30-plants-per-week#. .... but ref the Zoe 30+ supplement sold by Waitrose, it's cheaper to mix your own.
Regular plant food consumption cultivates a healthy gut microbiome which will assist regular bowel movements.
Regarding pooping at the same time every morning, i.e.,
at a very regular time:
Consider adopting regular sleeping hours if not already part of a your lifestyle.
Before nodding off to sleep, set aside 5 minutes or longer relaxation time with deep breathing and feel your whole body relax. Say / think to yourself whilst deep-ish breathing:
"My legs are relaxing .... my arms are relaxing .... my shoulders are relaxing ...." etc etc. You'll likely experience the feeling of your blood circulating through your limbs and other body parts as you 'sink' into a more relaxed state of mind.
In this 'more relaxed' frame of mind, it's possible to 'auto-suggest' to yourself, any aims and targets and positive thoughts – and thus 'achieve' same.
Try suggesting,
"I'm gonna have a very relaxed night's sleep and wnen I wake at 7am, I'll have a coffee and breakfast – and my gut will be ready for a its regular poop movement .... my gut will ready for its regular poop .... my gut will ready for its regular poop .... my gut will ready for its regular poop ..."
The repetitive auto-suggestion will likely work for you and you can reinforce same by saying / thinking to yourself ....
" .... and it's gonna work for me .... it's gonna work for me ... it's gonna work for me ... "
Auto-suggestion, positive thinking , self-hypnosis .... call it whatever you wish.
BW, dunk