You can try spreading them on the heater. That way they should get a little bit harder, but they will still be a little bit sticky. Like all old cards. I would not do it with very expensive cards because I do not know anything about the long term consequences.
normally when it becomes to soft for me is when they are beat to shit. so normally its just time to open a new pack, so ill probably donate the “too soft” one. sometimes they become to soft where they just lose all their snap, makes springs actually hard to do lol.