We cannot get KSM next Reset. The only thing we can still get is cutting edge for a week more.
What tends to happen is that we get a week of pre-season. As to when you lose your key level I am not entirely sure. According to RiO during post seasons I was doing relatively high level keys (19 and 20) back in BFA, so I assume that is how it is going to work here as well: You should still keep your current key level for a week more, and it will downgrade by 5 levels when season 2 officially launches.
I would go for safety and try and get a high-ish key this week if you can manage it.