On MacOS, new pipeline, added a $match stage, fine.
Went to add $group stage. Filled in field for _id <<expression>> now before doing anything else delete from comma back over the _id field. Every time it froze after the third character was deleted (so comma, closing quote and 'e'). See screenshot for exact entry.
Also attaching sample from Activity monitor - it seem to run at 100+% indefinitely (I killed it after 25 minutes the first time and decided it's never going to come back).