It's absolutely not too much or too soon. My physio just said to wait 10 weeks. I was scared about doing body weight exercises etc etc but my breast surgeon said all is okay and to go for it. So I have. I do some aquafit classes which works upper body also...good starting point. I do machine weights, kettle bells and dumbells... and I'm working on body weight exercises. I still really struggle holding my own weight (being really fat did not help) however as I've got smaller I can at least hold my weight for a plank. So start off light and increase as you feel ready. If you do it regularly you'll improve quickly.
I don't do boxing as it plays hell with my neck (even in water) but no reason why you can't. It will feel weird at first but you'll adjust. If sore the next day across the chest...don't freak out...its just using that chest muscle again . ..a quick massage will help.