题主要的是委婉地表达对吧?那为什么非要想着用could去代替should呢?委婉与否并不取决于用could还是should,而是整句话如何表达。
should是表建议,但是一上来就说should会有种指导别人高高在上的姿态,所以日常的表达为:
肯定:I think...should...
例如:I think you should take a shower.
否定:I don't think...should...
例如:I don't think you should work so hard.
疑问:Do you think...should...?
例如:Do you think I should buy this car?
这样说就有礼貌,如果直接说You should take a shower. You shouldn't work so hard. Should I buy this car?会显得你在命令别人。