Inspired by @ByteCommanders second comment, the script below does what you describe: assuming the downloads folder is the directory you download into, it disables suspend during download, and subsequently waits for five minutes (arbitrary to set) before re- enabling suspend, to make sure the download is really finished.
You can set any other directory to be watched as download folder.

How it works

In a loop (once per 20 seconds), the script checks the size of the targeted folder with the command:

du -ks ~/Downloads

The script compares each check with the last one, to check for download activity (increase of size). If there is no activity for more then five minutes (but you can set any other "wait" time), the script assumes no download is going in, and "normal" suspend is (re-) enabled.

The other way around: if the script sees increasing size of the ~/Downloadsdirectory, it disables suspend, until no activity was detected for more then five minutes.

Low on juice

The script's commands are extremely low on resources. Since the cycle runs only once per 20 seconds (as it is), The processor load is practically none.

The script

#!/usr/bin/env python3

import subprocess

import time



#--- set suspend time below (seconds)

suspend_wait = 300

#---



#--- you can change values below, but I'd leave them as they are

speed_limit = 0      # set a minimum speed (kb/sec) to be considered a download activity

looptime = 20        # time interval between checks

download_wait = 300  # time (seconds) to wait after the last download activity before suspend is re- activated

#---



t = 0

key = ["gsettings", "get", "org.gnome.settings-daemon.plugins.power", "sleep-inactive-ac-timeout", "set"]



set_suspend = key[0]+" "+key[-1]+" "+(" ").join(key[2:4])

get_suspend = (" ").join(key[0:4])



def get_size():

    return int(subprocess.check_output(["/bin/bash", "-c", cmd]).decode("utf-8").split()[0])



def get(cmd):

    return subprocess.check_output(["/bin/bash", "-c", cmd]).decode("utf-8")



check_1 = int(get("du -ks ~/Downloads").split()[0])



while True:

    time.sleep(looptime)

    try:

        check_2 = int(get("du -ks ~/Downloads").split()[0])

    except subprocess.CalledProcessError:

        pass

    speed = int((check_2 - check_1)/looptime)

    # check current suspend setting

    suspend = get(get_suspend).strip()

    if speed > speed_limit:

        # check/set disable suspend if necessary

        if suspend != "0":

            subprocess.Popen(["/bin/bash", "-c", set_suspend+" 0"])

        t = 0

    else:

        if all([t > download_wait/looptime, suspend != str(download_wait)]):

            # check/enable suspend if necessary

            subprocess.Popen(["/bin/bash", "-c", set_suspend+" "+str(suspend_wait)])

    check_1 = check_2

    t = t+1

How to use

1. Copy the script below into an empty file, save it as no_suspend.py



2. In the head section of the script, set the desired "normal" suspend time (since the script will re- enable suspend):

   
   
   

   #--- set suspend time below (seconds)
   
   suspend_wait = 300
   
   #---
   
   




3. If you want, you can set other values:

   
   
   

   #--- you can change values below, but I'd leave them as they are
   
   speed_limit = 0      # set a minimum speed (kb/sec) to be considered a download activity
   
   looptime = 20        # time interval between checks
   
   download_wait = 300  # time (seconds) to wait after the last download activity before suspend is re- activated
   
   #---
   
   




4. Test- run the script with the command:

   
   
   

   python3 /path/to/no_suspend.py
   
   




5. If all works fine, add it to your startup applications: Dash > Startup Applications > add the command:

   
   
   

   python3 /path/to/no_suspend.py