You can be in nat parks year round... Getting there is the only obstacle for some if the more remote destinations

Many of the state park campgrounds are open year round, as are the smaller national forest campgrounds. I just updated our camping map to show the campgrounds that closed in October. This is not perfect (sometimes it is hard to get information about seasonal closures) but most campgrounds have a link to the page: https://maps.northwestportal.com/camp/#8.00/47.408841/-120.273283 I agree that you want to stick to the lowlands and by late fall also probably want to stick to the western side of the state as Eastern Washington is significantly colder and snowier in the winter.

Unless the campground explicitly states closed, you can use it. You can also disperse camp

Dispersed/dry/boondock camping on NF land is the way to go. Just bring a small shovel to bury your poop.

Absolutely. Just stay in the lowlands and be prepared to get damp. Although this October has been one long freakish indian summer.

Check out the State Parks. National forest sites in the lowlands generally become first come first served.

Which Park(s) are you interested in?

I was also thinking about camping in/near Mt Rainier. Does anyone have good recommendation?

It's literally 80°+ right now in western Washington, camping season isn't close to being over