Mutt settings are mostly inscrutable to me, so I’ll just dump the ones I found (through trial and error) that do what I want below. Besides having these:
set sort=threads
set sort_browser=reverse-date
set sort_aux=reverse-last-date-received
I needed to add these:
set sort_re=no
unset strict_threads
For convenience, I also added these:
# Collapse threads at startup
exec collapse-all
# Set the keys for un-/collapsing all threads/one thread
bind index _ collapse-all
bind index - collapse-thread
(I’m using Mutt version 2.2.12 in Ubuntu 24.04.)