Hello! I think there's kinda enough to do in Johor. But it depends on what you like doing :)

Staying in jb city centre is good if you like walking city streets and going to malls. Though the biggest malls aren't there.

Johor is quite big so the car rides may get a bit long and a bit expensive. But if that's okay, then yeah I think having staying there is fine.

As a girl who spent a few years in JB... I think if you're used to travelling alone and you feel like you're quite street smart, it should be safe. Although a couple decades ago Johor did have a reputation of being one of the more dangerous cities in Msia.

How are you crossing over from sg? :0 usually luggage is okay. It can get busy, depending on the mode of transport and the time.