flat ironing and moisture retention help

I have 4b/4c hair and have been concerned about moisture retention. To combat this problem, I tried rollersetting my hair but that was a disastrous failure due to the fact that I couldn't get the roots flat/streched enough so they were sooo tangled and combing my hair dry and super curly is a no no.I did manage to get through it but I've said "never again" to the rollerset. However I noticed that my hair, this morning, is super soft and the ends feel much less brittle than they usually do. I think it's because my hair air dried instead of being blow dried and so ther was more  moisture in the hair shaft.  I also used fermodyl 619 from roux for the first time so that might have something to do with it but I don't know.  Does anyone have any alternatives to rollerseting for 4b/c hair for streching the hair before flat ironing?  I feel like I would retain more length and also maybe regain a little thickness since I have very fine strands with.  I'm also long term transitioning out of a pixie cut(17 mths) so have a few inches of relaxed hair at the ends and I'm about APL at the very tips but feel like I should be retaining more length than this and I wash and flat iron once a week with DC and my hair stll shrinks up to my head like a frightened snail when wet so i don't think I have heat damage.

There's two ways I stretch before flatironing. The first way is ponytail rollesets. Unlike traditional rollersets my roots come out flat/straight. The other way is to blowdry using the tension method. With this method no combs or brushes are used just pulling a section of your hair taught at the ends and running a blowdryer over your hair. To see this method look on yt for pinkskates blowdry method.

I think it helps to roll up when rollersetting because it pulls the roots flat and downwards towards the scalp and it's easier on your arms. And I agree- the tension blow dry method works really well.