Firstly, not used Asus supplied or even default firmware in 7 years or more on this router. In fact, there is security flaws in earlier firmware that were patched.

On my RT-AC87U, I currently use version 384.13_10 of Auswrt-Merlin https://www.asuswrt-merlin.net/ and I am about to retire this router for something that is much faster on wifi (which is all I use it for now).

You should always factory reset after installing new firmware unless you know for sure what changes are and what they mean.
Normal practice has always been, you just install the latest one, you do not need to move through each one like patches or something unless specified.
This router has been dropped from support by Asus thus unlikely to see new firmware for it again.