I use a couple of tricks in my own workflow…
Firstly when drying, I hang with a quite weighty clip on the bottom and let gravity do the work. When the negs are dry, then 9 out of 10 rolls are pretty flat.
If that does not work then I gently “adverse roll” (is that a term?) and then put them into a canister for a few hours.
If that does not work, then I cut and sleeve them in standard A4 pages and then adverse roll them around a cardboard Pringles tub for a day, holding them in place with some cardboard and elastic bands, making sure there is no tight band going over the negs themselves. When they come out they are as flat as a pancake.
After scanning, all my stips of negs are in archival binders, laid flat (not upright), so for re-scanning, flatness is never going to be an issue.
Hope that helps and gives a few ideas.