I hate to disagree with you on this, but having your knees higher than your waist is the best position. One way to help in this is to build a short stool (8" or so) that you can drag under your feet while seated to raise your legs if you have a comfort height toilet.That said comfort height will get things moving in the correct direction.
It's actually the opposite, ADA height toilets (17"-19") cause the strain and problems you speak of. They are easier to get onto and off of because handicap persons can slide onto them, and older folk don't have to push themselves up with their legs. But as far as digestive health the high height is bad. Only western countries sit with legs closed and knees lower than thighs.Most Americans dont know shi$%T and I mean that in that they dont know how to defecate properly- too much strain on the large intestine, IBS, Crones, hemorrhoids. Google Squatty Potty’s . That said comfort height will get things moving in the correct direction.