I don't know if it's a solution you'd like, but there is a bottom nav bar for comments. That's what I do for those really long posts. If you gimme a sec, I can find the setting link for you.

Edit: Settings shortcut: Comment options > Display navigation bar

Edit: what the hell is going on with that account that keeps spamming nonsense?

More than 600 comments in less than 3 hours? Probably safer to block and move on.