A lot depends upon your strategy and goals.
If your only goal, as implied in your post, is to “keep the interactions going each month once the people in the city have seen it”. Then you already know the solution: Just create new ad sets with new creatives”.
However, if your objective is something like conversions or commercial profits then you might be “shooting your self in the foot” metaphorically speaking.
When you understand how FB’s automatic optimization algorithms work you will realize that a drop n traffic can be a result of optimizations kicking in after an ad set has gathered enough data from the campaign to narrow traffic and improve on the campaign objective.
At the point, optimizations start to kick in your FB campaigns ROI is likely to start to improve, yet many incorrectly see the sharper focus on converting traffic as the campaign is dying. That’s because they are busy watching the wrong metrics and don’t understand what’s happening. If they identified and were paying attention to the key metrics that fit their business objective it is at this point you have an opportunity to begin gradually increasing your budget to cash in on the optimizations.
Again, it all depends on the primary objective of your campaign. If you are going strictly for social engagement, then you probably just need fresh ad creatives. But if you are going for a commercial objective, it might be time to increase your daily budget, and start scaling up.
If your objective is conversions then you can also look at scaling up by using a lookalike audience of your converted audience. That will allow you to scale up using the exact creatives that worked on a fresh audience list that closely resembles those that converted. This is the ultimate way to leverage your campaign data.