Planner decoration is basically taking planners and decorating them with tons of stickers and washi tape. There’s a massive YouTube and Instagram community around it. There isn’t really a way to make it to “best quality possible”, because it’s all up to your personal taste and also your access to particular stickers or tape. Feedback tends to be “ooh that’s pretty” which again is super subjective! So an achievements-based model would not work at all with a hobby like this.
Similar hobbies that don’t map out so well to skill-based achievements: any sort of collecting, anything that involves travel (one of my hobbies is to go to science centers, for example), reading, makeup/beauty, being a consumer of performances. I suppose you could make the “achievements” more of a How Big Is Your Collection thing, but not everyone collects for quantity, and then it ends up being “how skilled a buyer are you”.