Welcome Neocon! Have fun here.
If I may, here are my 2 cents regarding your complaints...
Not much to say here, it's completely subjective. Some people are going to love the avatars, others won't. I don't think they all look the same, but to each their own. As for the art style... well unless I'm mistaken they were all made by Ame, so it kinda makes sense they'd feel similar. Decibel best skin
That's true. It can seem especially odd given that Reborn intentionally avoids any mention of how much time passes between events. Were the kids left at the Meteor Daycare for a couple days, or for weeks? Did Tania survive on nothing but polluted water and Meteor blood for days? Who knows. I don't think these arcs are supposed to last very long story-wise, but gameplay-wise they do feel a bit long. Probably because the player generally won't go through all these events in one sitting.
They are, but it's also up to the player to be smart about defeating very strong foes despite being kind of underleveled (and in this game, the more you progress, the likelier you are to be). It adds up to the challenge, in a not very subtle but effective way. It doesn't bother me that new mons are obtained at a lower level, either. Being given a lv75 Metagross right off the bat would feel a bit cheap.
Field effect notes aren't required. They help, but it's entirely possible to beat the game without reading them ever which may or may not have happened to me. They are borderline necessary to fully explore a couple optional locations though.
I get what Ame was going for with the slow sand, but I have to agree that slowing the player down in Tourmaline feels a bit extra, especially considering how huge the area is. It's a mechanic that makes sense, but it's not a fun one. I suppose whether you consider it a good or bad thing depends on what you expect from a game like Reborn.
Regardless, I'm glad you have an overall positive opinion of the game and hope you'll keep enjoying it