@jwz it has always been surprisingly hard to crash JSON parsers, or even to get them to time out. See https://seriot.ch/software/parsing_json.html.
If attacking the parser is not an option, we are back to filling memory, which can probably be done by using really big strings. Presumably you do not want to store and transfer all those bytes, so you gzip them, and then it is just a gzip bomb with an extra layer: gzip bomb needs to expand into something that seems like JSON to the parser.