I was stuck in a daily hair wash cycle, and have very fine hair (but volumes of it).
I now wash my hair twice a week, and it's definitely improved the general condition of my scalp too.
Agree with a lot of the different bits of advice.
I only shampoo the scalp with my fingertips, and let the lather rinse through the remaining length to clean it. I never put shampoo anywhere but the scalp.
In reverse I never condition the scalp or first few inches only the ends.
The rule for conditioner I've been told is no more than a 10p piece blob in your palm. If you rub these together in your palms it goes a long way.
I try once a week to let my hair dry naturally without heating up my scalp. I do this before bed, and then straighten in the morning. I've stopped using the straightners close to my head. I think the heat brings out the oils tbh, bringing the lankiness.
The condition will come back if to can try and prolong the washes and let your hair find it's natural oil level I think.