I'm not a bib user, so maybe I can't help, but
for surprises, I have thin rain pants for hikers which can fold up small & get slid on quickly if conditions change from what I expected
but if I know to expect cold rain or wet snow, that calls for my hard core stuff. basically cycling specific storm pants (
by "Showers Pass" brand), under which I have various cycling pants to choose from. thin lined tri-shorts are always my base. between the shorts & storm pants, I choose a middle layer such as very thin running tights, or my regular cycling pants, or some thin fleece tights that I sometimes use as winter pajamas
they key for me, with dressing for rain, is what will the temperature be. I like to protect myself from the outside moisture (rain or snow, etc) but not at the expense of my thermal comfort, so choosing the right middle layer becomes my challenge
the fewer layers, the better. my Novara Headwind cycling pants are good enough in very light rain for an hour or so