I've found success since my initial post on avoiding conventional shampoo. I desperately searched the internet and learned two things:
1) Dr. Bronner's has two kinds of conditioner - a rinse and a leave-in. They aren't super clearly labeled on their site and I had purchase the leave-in, so using as a rinse wasn't working as well.
2) Hard water leaves soap residue in your hair. I definitely have hard water. The solution is an apple-cider-vinegar rinse. I really didn't want to do the baking soda/ACV thing but I don't mind just rinsing with the ACV.
Here's my new routine:
A. Wash my hair with the soap - don't be afraid to use plenty. It will make good suds if you rub it in your hands for a few seconds first.
B. Rinse out soap.
C. Pour a diluted ACV/water mixture over my hair. I keep this in a pint jar and it's about a 1-ACV to 5-water ratio.
D. Leave for about 1 minute and rinse.
E. Lightly towel-dry hair, then rub a squirt of leave-in conditioner into my hair.
F. Only finger-comb hair between washings.
As for F, I really just need to scrub my hairbrush. It got a lot of residue on it during my initial experimenting and I'm sure once I get it off each bristle, I can brush my hair again. I might just buy a new brush. I've had this one for probably a decade and it has lost a few bristles.
My hair now feels clean for 3-4 days (same as shampoo), is EXTREMELY tangle- and static-free, and feels delightfully smooth. I might dilute the ACV mixture a little more, because sometimes my head still smells like ACV for a day.
Next I want to convert the hubby to it. He doesn't have dandruff but has some other condition that causes his beard, eyebrows, and scalp to get flakes and irritated spots. Tea tree oil helps a little but I'm thinking losing the chemicals may help even more.