Thanks for the reply, Nicolas,

You might try higher humidity. It can take several days for
the film to flatten out.

Yes. Right now I am keeping it in a spiral container, wherein
the film remains laterally flat, in the bath with a hot shower
on. I was going to keep it that way for a while (so it absorbs
enough water) and then to start reducing the water flow, step
by step, to let the film dry slowly and uniformly, until I
switch the water off. Then, hopefully, it will be properly
dried and flat...

The other approach is to weigh it down flat. Put the film
strips between the pages of a book and stack more books on
top.

Just keeping them flat doen't help in my case. When I take the
films back they get curl again as if nothing has been done.
Some temperature and humidity conditions are probably also
required. And can one keep a high humidity if the film is
squeezed between pages of a book?

But I do keep all my recent films under weight. In this
regard, I got one more question: are transparent film sleevers
suitable for long-term storage? Or do I need those made from
acid-free papaer for that?

The best solution, vis-?-vis r.p.d, is a glass carrier, an
enlarger and a wet darkroom and make prints ...

Yes, yes and yes. But I do want to scan them. And the colour
process is way more difficult.

Anton