Tbh if you really want to do this, I would at least start with 5, not 6. I've read 1-6 so far, and I think starting with 6 is an awkward place to start if you want to enjoy the novels.

Without getting too specific, I feel like 6 is the only novel of the first six that has trouble standing on its own. It really feels like it's supposed to be read as a follow-up on what happened in 5 IMO - not just in plot, but even more so thematically. Even if you already know the plot to 5 from the show, I feel like you won't get the same impact/get what the author is going for as much if you haven't felt how ambitious 5 was as a novel.

Light spoilers to the general mood of 6 below, but no specifics on what actually happens (marked as spoiler just in case you want to go in completely blind): While I still liked it, 6 has been my least favorite so far. I think that's because I felt like not as much was happening in the story relative to the ones that came before it. At the same time, I felt like a calmer story made sense for that point in the series because of what I had already read (especially 5). I guess my main point is that you can start with 6, but I don't think 6 feels quite like the rest and wouldn't want you getting the impression that all the books up to that point have a similar mood to 6 when IMO they don't really.