Perhaps the issue is not that it cannot be said easily, but that for each thing you say, English might use a different expression. I mean one expression used multiple ways in Chinese might be multiple expressions in English & vice versa. I can only guess the difference between 大不了 & 没事, but I will try anyway. Slang: It's no big thing Street jive: Ain't no thing. (implies not a big thing) It's not special, I'm not going to be doing it. Whatever. . Sorry, I did not get the "not possible" thing.

nothing to lose (no big deal) , i quit 怎麼樣，老子就是不幹了 這是比較直接的說法(跟老闆對幹了) anyway, I think i am not suitable for this job 我想，我可能不適合這項工作(委婉地跟老闆說我想走人)

”没什么了不起的，大不了我不做了。“ 应该相当于说 “没什么了不起的，反正我不想做了” 吧。。（我也不太确定。） It is not a big deal, anyway I would like not to do it. not a big deal：就是没什么大不了的意思。 anyway：adv.不论以何种方式，不论以何种方法；无论如何，至少；但是；不顾 非喜勿喷> <