OK, so I got lucky with removing the header bolts, I used some Kroil and they all came out easily. One was already broken into the head but I could remove it by hand...
I found a used cat in L.A., here's the old and the replacement:
I installed it and now there is no rattle, yayyy!
I also looked at the efficiency of the new cat.
Looks pretty good to me.
Here's bank 2, for reference, also good:
In the rattling one the post-cat signal sometimes was clearly following the pre-cat's signal. Didn't throw a code though.
I uploaded a video of both here, just fyi:
https://www.youtube.com/shorts/wrMxLf7S0Rc
https://www.youtube.com/shorts/co477gS3rZE