Not sure if it's acceptable to link to other forum software, but have a look at Discourse @Ehren, it got infinite scroll and there are no issues with big threads, such as here:
https://us.forums.blizzard.com/en/wow/t/classic-era-patch-updates/1894672
I even found a 12 year old thread with more than 100k posts here:
https://forums.mixedmartialarts.com/t/the-sweet-science-the-ug-s-boxing-thread/2086472