My main beef with Wyndham was that he was too often derivative, often just making his own variant of, or polishing ideas other writers came up with (like for example Triffids vs Earth Abides). Midwich Cuckoos is quite original, though, but I found the actual presentation rather disappointing. The prose is weaker than that of Chrysalids, and the POV is the most boring one he could have come up with: a very passive male NPC. It would have been much more interesting from the POV of one of the pregnant women or even of The Children. Or just center it around Zellaby who was the real "hero".
Plan for Chaos looks like a rarity indeed; only two editions, and not Wikipedia entry.
Well, I can't say I found him all that derivative, and I think you need to be careful when you make that assumption ... especially back in that period of time, where it was a lot more work to produce a book.
Earth Abides (1949)
The Day Of The Triffids (1951)
Not much time in that age of snail mail, with authors living on two different continents.
And each is a very different approach to the other. Especially as one is about a virus and the other about a somewhat sentient plant going rampant due to biological manipulation and a global blindness event.
Each is a classic in their very own right.
Many books back in that period, had a certain sameness in some aspects, and a brave person attributes that to being derivative. That was not long after the end of the second world war, and many people would be thinking somewhat similar things.
And in any case, everyone borrows from everyone else, even if they don't know it. Wyndham was certainly unique where it really counted. https://en.wikipedia.org/wiki/Earth_Abides https://en.wikipedia.org/wiki/The_Day_of_the_Triffids
By the way, have you ever read the sequel? The Night Of The Triffids
by Simon Clark.
I quite enjoyed it. It gives a later American perspective. https://en.wikipedia.org/wiki/The_Night_of_the_Triffids