I think people overcomplicate this, not saying you, but I've sessioned for 15 plus years and always am paranoid about being clean going into sessions.
Wha I've learned for me, not saying you is a few things
1 - eat light the day prior, it's not so much about what, as when and volume.
2 - avoid alcohol and caffeine for 24 hours, maybe this is not relevant, but I do
3 - the DAY of session, after BM do a water enema, as many times until clear. I'm talking about the small squeeze 1's, not deep.
4 - then 2 hours prior, at least 1 more enema,
5 - all this the day of, then my 1st meal is en route some light crackers, and H20.
6 - lastly, if I feel "full" I also take empty enema to session, ready to use upon arrival for 1 last cleanse, and I do not think any Pro-Mistress would be upset with that to ensure clean.

A lot of that depends on the time of session, for me AM sessions are a NO go, and sessions after 8 pm take additoinal preps, but if I'm sessioning between 12-7ish I've had some 100 plus and never had a single unclean moment using this method. I can't recall which Mistress told me this before a session, but for me it was golden
Again, no clue what foods to avoid, other than the obvious spicy things, as it's more about your own schedule that you are already aware of,
GOOD LUCK