Controller over KB/M for me, hands down. If you have a mouse with additional mappable buttons, that could work for you as well. Note that the controller bindings on PC are a little backwards (O to select, X to cancel instead of vice versa like the default Playstation mapping) which led me to prefer playing on PS4 instead of PC.
For a casual player, there is definitely plenty of content available and I would bet that if yo consumed it casually enough, it would be a very long time until you run out of things to do. Also, I'd say the game is pretty forgiving of subpar comps in the beginning. You certainly don't need to reroll for 5* characters.
At some point, probably around the time you get into the second area (~AR 20?) you will feel it if you are missing a component in your party, like the correct elements to break certain shields. Painful, but not impossible. So that kind of incentivizes you to tune your comp.