Turns out having Claude test things in Chrome on it's own, made copies of Chrome in a temp file that were never deleted. The path was this:

/private/var/folders/c3/6s_l3vn96s5b8b9f08szx05w0000gn/X/com.google.Chrome.code_sign_clone/

I deleted all the temp files in here and got back 200gb of space.