Is saying hao3 ch1i ba rude? I already know the various Chinese phrases for "bon appétit" such as "chi1 hao2 he1 hao2" or "man4 man4 chi1" but they either seem to formal or like someone's scolding you to eat slowly. Is "hao3 chi2 ba" a legitimate way to express the sentiment?
The most common way to express bon appétit in more formal situations is:

请慢用

请[qing3] = please

慢用[man4 yong4] = Enjoy your meal! / Bon appetit!

If you find this too formal you may take off the 'please' (请) and just say: 慢用.

Mind you this is most likely going to be said by a waiter/waitress in a restaurant or someone who is not joining the meal.

慢慢吃

is an option, but also usually said by someone who has finished eating before the rest of the party. 

慢

Although, 慢 does mean slow(ly) in general in phrases like 慢点 (take care/be careful) or 慢慢X (慢慢走(take care)、慢慢吃(enjoy!), etc.) it doesn't, literally, mean slow but to take your time.

好吃吧

好吃吧 is made up of two parts 好吃 + 吧 —or— tasty + (I) guess. So this phrase really means - (It's) good, I guess.