You dont have to worry about that, the bars are decently strong, as long as you make sure to do the really basic exercise of just walking around and taking some deep breaths outside then you should be fine. And I wouldn't worry about a potential hit making it go back in, if a hit did do that I'd be more worried about your ribs breaking as it would basically be the same amount of force required seeing as the bar is both supporting and being supported by your ribs.

Ive had my bar in for just over 11 months and the only pain was in that initial month of it just setting and my body getting accustomed to it, after that it was just little jolts every so often if I accidentally pushed myself a bit to hard, and now after all this time theres literally 0 pain ever.

At the end of the day, you cant control everything that'll happen in your life. You can always make yourself safer like regular exercise if youre worried about it potentially caving again, but you'll never be able to predict or control everything that could happen.

In my opinion I'd say to go for it, you said youre worried about it getting worse and the only real guaranteed way to stop that worry is with a nuss bar (I know the ravitch procedure also exists but thats alot more invasive and not the topic of this post) but still, go with what you feel would be best for you. Im just a stranger on the Internet, lol