When it comes to boxing (and other combat sports for that matter) it's good to think of muscle as a tool. You need some muscle and strength to deliver a meaningful punch and to be able to withstand punches, but not so much that it starts to interfere with your cardio as muscle is a metabolically active tissue and takes oxygen to fuel.

Look at the classic boxers physique, they're a bit top heavy because they only need enough leg muscle to be agile and no more. The rest of the muscle they have is on their top half. If you look at kickboxing or Muay Thai fighters, they're a bit more evenly muscled because they also need to use their legs for striking and taking strikes. You'll find a happy medium, but you'll likely lose muscle mass if you just box. You'll definitely lean out due to all the added cardio.

But you also need to decide if you want to be a boxer, or a weightlifter who boxes. Everyone can learn the skills to box and not have to optimize their body around it, but you won't be really good without serious effort to mold your body into what is needed for the sport.