shinydashboard: execute data import x seconds after pressing a button - r

Question

BACKGROUND
I'm building a shiny dashboard in Data Science Studio where a user has to choose a file they want to process from the selectizeInput and then press Submit actionButton. Once the button is pressed, it starts a backend process to determine what category the file falls into and what workflow it should follow (different workflows contain different cleaning steps and final outputs). Once the workflow completes, I want to display the final output in DT::renderDataTable.
DESIRED OUTPUT
I would like my shiny app to
show no data before the input is chosen
time, let's say, 30 seconds from the moment someone clicks the Submit button to actually picking up the dataset
MINIMAL WORKING EXAMPLE
How could I implement it in this simple app?
UI
library(shiny)
library(shinydashboard)
library(DT)
library(dataiku)
path <- dkuManagedFolderPath("receiver") #folder with the files to choose from
dashboardPage(
dashboardHeader(title = "Catchy Title"),
dashboardSidebar(
selectizeInput("file", "Select File",
list.files(path)),
actionButton("submit", "Submit")
),
dashboardBody(
fluidRow(
box(h2("Show Data"),
div(style = 'overflow-x: scroll', DT::dataTableOutput('mytable')),
width = 12)
)
)
)
SERVER
library(dataiku)
library(shiny)
library(shinydashboard)
library(DT)
library(dplyr)
shinyServer(function(input, output) {
file_name <- reactive({
req(input$file)
})
####
# run the process that takes the file name to pass it to
# the global environment, which triggers the conditional workflow
# and may result in different data outputs
####
## AFTER the process runs (approx. 30 seconds after pressing the submit button)
# I want the shiny app to follow this logic:
if(is.null(nrow(try(dkuReadDataset("intermediate_dataset"))[1]))){
df <- dkuReadDataset("final_data1")
} else{
df <- dkuReadDataset("final_data2")
}
output$mytable = DT::renderDataTable({
df
})
})
Thanks for the hints!