Well for starters your on the right track, you keep correcting yourself even though you slip into the hunch. Your conscious is picking it up and your correcting yourself on the fly, just keep correcting yourself and be conscious of your posture it will happen for you.
Some atest that a backrest will do the trick but I disagree you can easily slip into a hunch with a backrest too, unless you lasso your torso to the backrest.
Imo there is no easy way to learn this, except to keep correcting constantly and eventually down the road you will maintain perfect posture all the time.
Also, learn to be aware of your posture off the set, especially when sitting at a work desk or at a meal. Learn to keep a straight posture at all times, whatever you're doing. Not only will it translate into your drumming, you'll avoid a lot of back problems in general especially as you get older.
When i play my drums in the evening, the windows reflect like mirrors and i can see my posture sideways, this helped alot, so perhaps you can drum with a mirror. It didn't just help me for my back (i slouch), wrist and stick holding too.