I just read fast over the device section and picked up the link to put your attention to possible clips/difficulties on this device.
I've read short over this thread:
1st: You have two options to flash this device. Either open case and use serial connection or use the "trx" approach. As I assumed you don't want/can't open the case I picked the trx section (Easy Installation). There was this warning from above what I just linked.
2nd: You can save the mentioned partitions/files from console usually.
But I cannot see if standard firmware has ssh access to save the files to an usb stick.
I assume it has not and the link is probably missleading if you come from stock firmware. So you have to verify it by yourself if you can access over ssh with stock firmware.
3rd: There is another link with more detailed description:
http://lede-ac58u.zyxmon.org/HowToFlashENG.html
I strongly reccommend using Method 1.
4th: I have no clue how to get from LEDE to recent 19.07 release. It is possible that there are another clips to upgrade (because of the age). Also it is highly possible that you can use the 19.07 uImage.tib file to flash from stock directly (from stock gui) nowdays. But this has to answer someone who know this device. If no other is joining this thread who has knowledge about this specific device it might help to open a new thread with like "Howto flash OpenWrt to Asus RT-AC58U properly?" or "I need help flashing Asus RT-AC58U properly".