How do I Wrap Functions Presented by plumber for Work in Promises

Question

I have about 20 typical functions like:

echo_delayed <- function(j){
  jj <<- j # for debugging purpose
  Sys.sleep(1.5)
  print(paste0('echo_delayed - ', now() %>% toString(), " ", j))
}

All of them have j parameter that is an embraced JSON string parsed in these functions. The functions produce different kinds of outputs. Typically ggplot()... %>% print(), string or byte stream. These functions are presented by plumber. For instance:

#* @post /echo_delayed
#* @serializer contentType list(type="text/html; charset=UTF-8")
echo_delayed

Using this sample I am trying to wrap some of them to work in a non-blocking mode:

time <- function(name_, name = name_) {
  paste0(name, ": ", Sys.time())
}

new_promise <- function() {
  promise(function(resolve, reject){ resolve(NULL) })
}

add_echo_delayed <- function(p) {
    p <- then(p, function(value) {
      echo_delayed(value) 
      "" # return value
    })
  p
}

#' @post /echo_delayed
#' @serializer contentType list(type="text/html; charset=UTF-8")
function() {
  new_promise() %>%
    add_echo_delayed() %...>%
    time("echo_delayed")
}

However it is not running in a separate thread and other queries are waiting until echo_delayed() is done with calculation.

The functions from above mentioned sample (with minimal edits) are working fine:

sleep_count <- 5
add_async_sleep <- function(p) {
  n <- 20
  for (i in 1:(sleep_count * n)) {
    p <- then(p, function(value) {
      Sys.sleep(1/n)
      "" # return value
    })
  }
  p
}

#' @post /async_post
#' @serializer contentType list(type="text/html; charset=UTF-8")
function() {
  new_promise() %>%
    add_async_sleep() %...>%
    time("async_post")
}

It stops working if I break splitting the sample function into pieces:

add_async_sleep2 <- function(p) {
    p <- then(p, function(value) {
      Sys.sleep(sleep_count)
      "" # return value
    })
  p
}

I am stuck. Could anyone please help me to properly wrap the function echo_delayed() so that it starts running in a separate thread and the parameter j is passed into it?