The iPad mini 4 is the issue... Usually it is causing issues for people after upgrading to iOS 15.2. There are lots of discussions about this on here lately...
Download the beta version and give that a try. We have added some iPad mini 4 CPU reduction measures that will get through the big bursts that are causing issues.
Look for build 010720221.
But hey, that's why we have the subscriptions: to keep up with the changing environment.
You should look into upgrading though. We have been recommending the use of at least an A10 process for over 2 years.