Hi, new guy here! Is there a spot to sign up? I'd like to join!
I've done the southern part of the WABDR twice, last time was last summer. I didn't see much group sized dispersed camping until I hit Bethel RIdge, but Council Lake (46.26281, -121.63219) is one of the nicer free campgrounds that I've seen. We stayed by Bear Lake just off Bethel Ridge (46.7275, -121.019) as well - a good dispersed option.
For the Subaru - everything should be easy enough until you hit Bethel Ridge, and at that point there's a decent bypass to avoid the deeply rutted & rattly main trail. Only other potential troublespot before Ellensburg was around (46.836,-120.8983) iirc and here I'm not aware of any bypass. I haven't done the Ellensburg to Cashmere section, I've been advised against it. Both those sections I mentioned might be doable with the Subaru, but a trailer behind it could be a challenge.