Ah yes...that fic...I've read it, but a lot of the reviews tend to be the same people spam posting. That said, I don't think as many people read high school AU fics as you think they do in the Pokeani fandom. I find the most popular ones for Amour have the adventure flavor going for them such as mine, FanaticLaguy's, AmourshippingCanon's, potat's, etc. The problem with his story is a lack of conflict. The Amour is strong; lots and lots of Ash and Serena moments...but not much else.
In regards to ratings, romance and Amour: never take a fan site rating as gospel. TV ratings and "what I think of this" ratings aren't the same. In either case, you can always expect them to inevitably dip as it has every saga since. Perhaps Serena's return to fuel the Amour could spike ratings, but no one could say for sure.
Have to agree with you about that story. It was rather well done, but lacked any sort of conflict. Nothing happened in it at all to try and cause tension or a plot. It just turned into a generic story where everything went right for Ash and Serena. I'm not trying to say it's bad, it was good, just no conflict made it a little boring to me.
As for other stories there are a few I've read in the past:
-An Unexpected Union: A rather older fanfic (published way back in 2014, was completed January 2015) that changes a few events following Ash winning his 2nd Gym badge. The chance encounter of running into an old friend changes his life for the better. Yes, this is Amourshipping.
-Snowbelle of the Ball: A one-shot that is a good read. Though, it made me sad with the ending. Not that it was bad, but because of how it ends. Pretty much the group is in Snowbelle City when a winter festival/dance takes place and they decide to go.
-Lights of Laverre: A slight change of events in Laverre City. Still an excellent read just because of how the characters are written and the overall plot.
-Painful Loss: Another one-shot that is kind of sad but gives a different look on their relationship.
And of course:
-Ancien Series: Triology of stories along with a handful of one-shots. Very well done and exciting read.
-Lumiose University: Still being written but well done. Ash and friends go to a Pokemon school, not a typical AU high school fic.
-Gotta Catch that One: Long story that has a change from the MAster Class on but it is a fun read. Would recommend.