You've probably done some churn analysis at this point -- do you know what % is passive churn (i.e. payment-related) versus customers who are explicitly canceling?
I'd consider spreadsheeting the biggest leaks and getting super aggressive with multiple approaches. Empower your team, consult with professionals, leverage third party tools, outsource... all out offensive.
It's difficult to know the ONE THING that will increase retention, but improving many things by a percent or two will give you noticeable lift.
And by compressing it into a short timeframe, you'll reduce the number of variables around cohorts, seasonality, etc.