Quick comment on biome selection:
For Creative Worlds you can select whatever you want. Any number of biomes. Any distribution of biome levels. We make a recommendation so that the world is varied. (But if you only pick 1 biome, then the world will be a single region!! Pretty boring.)
For Survival Worlds you’re asked to pick a distribution of biomes throughout the different Biome Levels. This is mainly to avoid players purchasing high level resource worlds and then filling them with low level Biomes, in essence making the world easier to harvest. (I think this was unavoidable to not hit claims of P2W.)
In either config you can pick as many biomes as you want over the required minimums. But these will still be distributed by the required rations. (And too many biomes will make the world thematically noisy.)