I reread it a couple months ago and I still think it deserves to be considered a classic, although it’s not really a book with narrative momentum and it’s not my favorite of his books. During his life it was the book that got the most recognition. Don’t you dislike Dick in generally anyway?
I agree that it’s at least comparatively not all that representative. The book that most resembles it, at least of the ones I’ve read, is The Martian Time Slip, which was the next book he wrote and the second-to-next book he published.
You could debate what counts as is first “mature” book but I think High Castle is the first one that definitely qualifies. I haven’t read the two that were published right before it, but I think people agree that they’re mature works, and they were written in 1953, according to this Wikipedia page:

https://en.wikipedia.org/wiki/Philip_K._Dick_bibliography

although I assume that if they were written in ‘53 and published in ‘60 then there was some kind of revision in between. They probably talk about it in The Dickheads Podcast, which is a great podcast, but I’m not listening to those episodes before I read the books.