1 million posts ain’t that big in the grand scale of things.
It’s over 1 million on the bike forum alone.
Tiny!
German MTB forum is pushing 8 million, and a fraction under 200,000 members. Now that’s getting a bit more sizeable in the grand scheme of things, but I’m sure there’s much bigger out there. ANd in true Germanic style has to regiment everything into 100 sub-forums so you realistically have to hit “show all new/updated threads”, and get it in a format essentially the same as STW 😉
VBulletin is pretty well supported, but not without its (stupid) faults (many of which I fix c/o greasemonkey, due to slack admins that cba to fix those faults), but it costs money (not sure how much).