The forum I was an administrator of for many years actually had it so that new users who had less than 15 posts were put into a "newbie" usergroup. Rather than move every post they made to the approval queue, only certain types of content would get flagged, namely URLs and certain keywords used by spammers and bots such as "Skype" and "Telegram". Posts by newbie member would also get flagged and sent to the approval queue based on matches from regular expressions. This seemed to work out for the most part as bots would rarely get through compared to how many were sent to the approval queue to be dealt with.
Once a member attained 15 posts, they were automatically moved to a "Newbie+" usergroup where most of these restrictions no longer applied.
I hope this helps give you some ideas on what you can do to limit bot invasions!