This is super weird, now I’m getting fewer words back (but still thousands) using:
.../_all_docs?startkey=%22_design%2F%22&prefix_match_level=1
I tried the other method, but fixed the escaping (my bad!)
.../_all_docs?startkey=%22_design%2F%22&endkey=%22_design%2F%EF%BF%BF%22
Which seems to work.
Thanks