Hi. My name is***** you for your question and the opportunity to assist.
If you do a reset, it will indeed erase and configuration changes you made to the router
and they'll need to be re-entered. If you created different wireless network names that those that came
with the router those will need to be put back in as well as the passwords for those networks.
If you've configured any port forwarding that would need to be set up again as well.
On this link you sent: https://www.asus.com/Microsite/2015/networks/routerfirmware_update/
it shows an AC68 router. Is that the model you have?
Thanks. I have an AC1900 myself. You can upgrade the firmware without doing a factory reset afterward.
It's recommended, not mandatory.
Here's a list of the things the latest firmware update for my router did (see attachment below)
Not a single item in the description affects me. Some of those things I don't even know what they are.
My point being that just because a firmware update is available, it may provide no benefit. It won't make my router
(or anyone else's) faster or last longer.
I did the update anyway but I certainly didn't need to and like in the past I didn't perform a factory
reset after the update. When considering a firmware update it's always advisable to go to the Asus website
and take a look at just what the firmware update accomplishes and make the decision as to whether you
need it or not.
Here's the link for your router: https://www.asus.com/support/Download/11/2/0/84/LzShv8ma7TrQB4eO/30/
Click the Firmware link and it will show the firmware downloads and give a description.
Unless you're having a problem that one of the fixes rectifies, I don't see anything there that's a "must have"