I dunno. Like the thing is you're never going to know everything, at some point you have to stop researching and start writing.
Like knowledge is fantastic but I think writing historical stories about areas you're not a complete expert on is fine. Writing gets you into the habit of writing. It doesn't have to be perfect to be worth doing. And honestly a lot of published historical fiction isn't particularly vigorously researched.
The problem isn't so much sitting down and writing Fatherland without a complete knowledge of Nazi agriculture but sitting down and writing a faux academic timeline about a surviving Nazi Germany without a complete knowledge of Nazi agriculture. The timeline structure is about showing off facts and thus rather requires facts because there's no narrative or characters, which doesn't mean it's bad, I enjoy just chatting about history but the knowledge requirement is high for it to work.
Saying the answer is more research is I think incomplete, so much as the answer is picking a format that fits the amount of knowledge you do have. AN actual story does not require as much knowledge because it's about characters and narrative.
Forums like AH.com have gotten into a culture where they expect timelines to both cover the entire world and every topic of humanity within in while also being primarily about well researched historical facts and obviously that's impossible.
You don't need to know the entire world before you write something, you just need to either only write that sort of dry chatting about history style essay about stuff you do know or write in a different style where your knowledge isn't front and centre.