Start reading manga... which level ? which manga?

Ah, I see what you mean… and completely agree!

The one I picked up is 小学 level, perhaps just a bit earlier. It ought to be easy, and doesn’t have the challenges you describe with a book for toddlers, but, with no kanji to cling on to and no frame by frame pictures, it’s harder than Yotsuba, and, being a story rather than facts, harder than なぜ?どうして?. But still, something fun to read on the train!