I am not so sure that it makes much sense to report spam posts that are few years old. There are plenty of new spam posts created every day that are reported, so if you report posts made in 2015/2016/2017 it will just put some extra pressure on mods, that are already swamped with new reports.

Might be the best to simply focus on recent and new posts, and if you really have a lot of free time and want to keep reporting spam,  you might check some other boards too.