Since you’ve started しろくまカフェ, I do recommend completing it first, if possible. The only thing that should stand in the way of finishing anything you start is if you don’t enjoy it. Enjoying reading something is a good motivator, and is instrumental in learning. But aside from motivation, which is fleeting, you want to ensure you have a schedule to keep up reading. That’s where following an active book club is nice, because it helps provide a schedule/structure for dicipline.
As for which manga for grammar beginners, I’d personally recommend:
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よつばと！: You’ll see this one recommended a lot. It gets a bit easier starting with volume three. However, I don’t necessarily recommend skipping the first two volumes unless you’ve already read them in your native language. Regardless of where you begin, you’ll encounter a lot of casual language that text books don’t prepare you for. That’s where having the book club comes in handy. (This one was actually a Beginner Book Club read, but that’s probably because the Absolute Beginner Book Club didn’t exist yet at the time.)
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三ツ星カラーズ: I like to describe the difficultly of this series as “よつばと！ without the easiest or hardest chapters.” There’s no book club for this series, which I count as a negative compared with series that do have book clubs here.
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レンタルおにいちゃん: I find this series does a nice job of starting slow with simple grammar, then ramps up the difficulty at a very slow pace throughout its four volumes.
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Teasing-Master Takagi-san: I’ve only read the first three or four chapters, so I’m not yet been able to ascertain the overall difficulty of the first volume of this series. But we’re going to read it in the Absolute Beginner Book Club starting in about five weeks. I highly recommend considering it simply because you’d get to read it actively with the club.
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それでも歩は寄せてくる: This one is nice because chapters are typically about six to eight pages long. There are a number of Shougi terms used, but these are easy to recognize as Shougi terms when looking them up in Jisho or another Japanese-to-English dictionary.
For learners of Japanese who are just starting to build up their grammar, any manga you read will be difficult to understand, no matter how easy the manga is. And it will take a lot of time to get through. This is why I say to do your best to finish what you start. Be sure to utilize the book club threads (when available) to their fullest, including asking any questions you have!