Can I get Akka HTTP to backpressure when it encounters an error?

I'm using Akka HTTP in a project and for certain flows, we have external infrastructure that can easily become overloaded, throwing back HTTP 500-level errors when we do POST requests (which in our case are idempotent). In these instances, we retry the request like so:

    RetryFlow.withBackoff(minBackoff = retryMin.seconds, maxBackoff = retryMax.seconds, randomFactor = 0d, maxRetries = numRetries, httpReqRespFlow)(
      decideRetry = { (request, response) =>
        response.httpResponse match {
          case Success(httpResponse: HttpResponse) =>
            if(httpResponse.status.isSuccess()) {
              None
            } else {
              processHttpResponseFailure(request, httpResponse)
            }
          case Failure(exception: Exception) =>
            logger.error(s"Retrying HTTP request from ${response.httpResponse} future failure", exception)
            response.httpResponse.map(_.discardEntityBytes())
            Some(request)
        }
      }
    )

   }.log("Backoff HTTP request")

I understand that Akka HTTP will backpressure if the entityBytes are not consumed or discarded but is there any way for me to force the flow to backpressure when we get these load-related errors? There will be several streams of execution, and ideally, I'd like the whole system to "throttle back" if it starts to overload the downstream system.