I don’t think food matters. The key is LOTS of water.

With food is better I think. The fiber doesn’t get mixed with all the food you eat during the day if you take it all at once, and it’s better to get the fiber spread throughout your daily food intake.

Ok, but I just found this:
https://www.berkeleywellness.com/supplements/other-supplements/article/psyllium-and-nutrient-absorption. Don’t take it with medication is the important thing. last paragraph:
“Note: Psyllium may interfere with some prescription drugs, so talk to your doctor or pharmacist if you take psyllium and a drug at the same time. Labels on psyllium usually say to take it at least two hours before or after medications.”