Frankly I think Super Mario 3D World is the right kind of difficulty for how I play it, but I can see where some gamers might need more of a challenge. If you are a challenge seeker Super Mario 3D World might not be for you but having kids might help you get there.
Oh and we haven't played Bowsers Fury yet but I imagine we'll get there after we complete Super Mario 3D World. We just got to the mid point in World 3 even if the game feels like a continual hunt for the kitty suit.

Is Bowsers Fury a completely separate game?
Yep. You need to buy Super Mario 3D World on Switch and it runs on the same engine but it's a totally separate thing you select at the menu of the game.
Rad, it's like old school SMB/Duck Hunt lol