I’ve read Ｄ坂の殺人事件 and I think it would be a good one to test the waters with, since I think it’s full-on for adults, but it’s a short and fun mystery and an introduction to Kogoro Akechi who you could follow in plenty of other stories if you enjoy that one.
I don’t think the grammar should be too much of an obstacle if I recall correctly… Usually with Ranpo I think of the descriptions as pretty straightforward, but the vocabulary and kanji usage as musty and a little hard to get used to at first. So I think if you’re used to the author and are hungry for kanji it should probably be a fun time!
And you could give the current book club pick a try 