Every now and then I hear here the same thing " set it up minimally and manually without using a previously saved backup config file". Why after the reset I cannot use the config file? Isn't just a file to "fill" the preselected options? In the end, it just a file that has some predefined values. Same as you would configure all the options manually?
The issue is that some of those predefined values may have changed meaning or context with the new version. Or may be obsolete, completely, (depending on how old of a firmware you're upgrading from).
The options often change. The variables often change too. Both may change from their original (first) use. Variables get reused and recycled. Their meaning changes from firmware to firmware. Not only that, but helper scripts may have a completely new method (internally, under the hood) to offer the same external feature.
In addition to all the above, there isn't much testing (if any, I believe) done from upgrading from all the possible combinations of all firmwares available, for all models.
In addition, certain specific hardware identifiers are also contained in the config file (that is why you shouldn't use one config file created on another router, on the same model router either).
The number one reason to do a full reset with an M&M install is because of how fast it is (10 minutes or so).
The second number one reason is that a full reset followed by an M&M install is it gets the router to a good/known state (see reasons above why it's not there already). From there, true troubleshooting can begin, rather than just simply chasing wild geese (if the issue does stem from the possibilities above).
The information above (and the links below) has been gathered, followed, and proven to me from various posts and via personal experience with many Asus routers, over the last decade from this forum.
Do with it what you will.
Almost all L&LD Links
About L&LD
Fully Reset / Best Practice Setup / More
That isn't the recommended method anymore. Not even by Asus
Why RMerlin vs. Stock for Security
RMerlin Firmware Reasons over Stock