This is the critical point. You have this line:
There were 132969 unused item pointers.
Which says there's 132k or so dead rows in your table. Which means
vacuum / autovacuum isn't keeping up. Did you try and stop the update
several times? Each time it starts then gets killed it creates dead
rows.
Try doing a vacuum full followed by a reindex OR a cluster on this
table and see if that helps.