My solution was as follows:

My logout method after successfully logging out, redirects the app to the /login route:

@Injectable
export class AuthService {
  // ...
  logout () {
    // do logout
    this.router.navigate(['/login']);
  }
}

My guard is then as follows:

@Injectable
export class DiscardChangesGuard implements CanDeactivate<MyComponent> {

  canDeactivate(component: MyComponent, currentRoute: ActivatedRouteSnapshot,
                currentState: RouterStateSnapshot, nextState?: RouterStateSnapshot): Observable<boolean> | boolean {

    if (nextState.url === '/login') {
      return true; // bypass checks if we are trying to go to /login
    }

    // perform regular checks here
  }
}

Here's how you can force a bypass on a per-navigation basis. In my case I was navigating back to the account dashboard after the user submits a form with account info.

Ideally you would check for unsaved changes in the form and bypass based on that, but my forms don't have that feature yet.

form component:

this.router.navigateByUrl('/', { state: { bypassFormGuard: true } })

router guard:

@Injectable()
export class ExitFormGuard implements CanDeactivate<ComponentCanDeactivate> {

   constructor(private router: Router) {}

   canDeactivate(component: ComponentCanDeactivate): boolean | Observable<boolean> {
      if (this.router.getCurrentNavigation()?.extras?.state?.bypassFormGuard) {
         return true
      }

      // perform normal checks
   }
}

My goal was to bypass route guards without touching every route guard in the app, but I was unable to do that. Ultimately I created a new RouteBypassService to inject into every route guard. RouteBypassService implements shouldBypass, which returns true if the route guard should allow navigation for some overriding out of band reason, such as authorization. In my case shouldBypass returns true iff there is no user signed in. (After they sign out, we let them get off the screen no matter what).

It's not ideal because every author of a route guard has to affirmatively remember to add the bypass, but it's not terribly entangling.