You're correct, it's not a prerequisite. But Girls' Last Tour did actually get an official English release before the anime was even announced at Anime Expo 2017. It's also generally (not always, but generally) a good indicator that a manga is more likely to be adapted into an anime these days. I personally don't believe they'd adapt Shjimeji Simulation into an anime before putting out an official English manga release.
No. There aren't even any plans for an official English manga release yet.
If I can dream big, I'd like them to finish adapting Girls' Last Tour somehow, anyhow, before doing a Shimeji Simulation anime in the future. And not just because it's so beautiful that it deserves finishing, but because of how relevant GLT ended up being to Shimeji Simulation...
He tragically passed away on June 9th. https://twitter.com/POCBooking/status/1535738833311916032
Really devastating. RIP.
It's okay if you're fine with the Netflix model of consuming content, and Netflix isn't entirely devoid of its upsides for both streaming and distribution despite being a huge corporation that primarily cares about money over its users. But the social and communal aspects of anime watching and discussion, particularly around an anticipated show, particularly around one historically released with a normal anime release schedule, shouldn't be dismissed so easily out-of-hand. This is especially true when you can objectively gauge the way said anime is searched, talked about, and kept up with. All you need to do is use Google Trends or spend more than a single minute scrolling through Twitter search results. You can even see the forum threads on this very site made for Part 6 in comparison to previous parts and their level of discussion to see such an impact.
Another side-effect of this nonsense Netflix is pulling is how terriblly anime database sites always get impacted. Don't get me wrong, for anime that isn't released like this, database standards are perfectly agreeable and understandable. But when it comes to say, Stone Ocean, Alicization, or any other numerous anime that is a "two+-part" show that's only really split up into multiple shows by virtue of airing a couple seasons apart? It's maddening. It's the limited best anime database sites can do with the current standards applied to anime released on a normal schedule, but feels its flaws in these situations. Stone Ocean is obviously produced and meant to be one show, and almost certainly wouldn't be subjected to such a terrible release schedule and way of promotion if it wasn't for Netflix. (Even though Netflix isn't always to blame, like with Alicization or the new Legend of the Galactic Heroes stuff.)
Still plenty excited for more episodes, because Stone Ocean is my favorite part, Jolyne is my favorite Jojo'a protagonist, and the main villain is also my favorite in the series. If the last set of episodes is any indication, there is almost zero chance David Productions could fuck up any aspect of the rest of the adaptation, given the absurdly high quality. But man, Netflix really fucked up the release, anime sites have to categorize it weird, people might not talk or remember it as much when it finishes... a real shame!
They eventually took it down after making moves to be more of a legitimate platform for anime & manga, but people could leave comments on an anime's page for subgroups that did the show, and those comments usually devolved into flamewars and slurs being thrown around. I'd say maybe 5% of the time you'd see a useful comment about the quality of a subgroup's subs, if that much.