You are often getting into the conditional -NGE- and -NGALI- tenses when the word "would" appears in the English sentence:
Ingekuwa heri kama wasingepika/hawangepika - It would be better if they didn't cook (and several other possible translations, but basically it needs to be a condition still possible of realization)
Ingalikuwa heri kama wasingalipika/hawangalipika - It would have been better if they had not cooked (and several other possible translations, but basically it needs to be a condition no longer possible of realization.)
To me it seems like the straightforward translation would be "It's better they not cook" (and you can add the "that" if you happen to feel like it). The "should" seems like a way to get around using the English subjunctive. But the subjunctive is common in American English, and the "should" doesn't sound natural to me. I'm curious if it sounds natural to others? I've read that "should" is one of the common British alternatives to the subjunctive.
I agree that the most straightforward translation(s) stick closely to the original subjunctive construction: "Better they not cook" or "It's better they not cook." However, I would say most speakers of modern English tend to avoid the subjunctive, except for some common constructions, and that leads them to use conditional tenses of various sorts. This means there will be a long list of potentially acceptable translations, as people employ their favorite way(s) of avoiding an actual subjunctive construction in English. And there will be a lot of disagreement about what is and is not on that list.