If you are just saying entering JB from SG, you can just take buses in then use grab to travelling around Johor, but if you wanna explore the rest of Malaysia, then maybe hire a driver!
I’ve been to Legoland a few times and I don’t think the queue were long as compared to USS. Even during pre covid Christmas and this was Hari Raya 2022. One day should be enough to cover everything (no kids was with me).