Just want to jump in to say that “environmental determinism” can make a world seem kinda flat. So like you said, don’t get bogged down by the nitty gritty details too much. However, it’s good to learn the rules and norms of worldbuilding to gauge which one actually matter to your story. And more importantly which rules you want to break.

So yeah, rivers always flow into the ocean. But maybe there’s a reason why some of your rivers work the opposite way.

Edit: By the way I really like your flag designs!