What if you only use official Apple AC adapters?
I have at least 2 dozen iOS devices at work (an assortment of iPads ranging from iPad 2 all the up to iPad Pro 12.9" 2nd gen, iPod Touches 4th gen to 6th gen, iPhones from 3G to XS Max) + my own home devices (iPad mini 2 that I bought years ago, iPad Pro 10.5" that replaced an Air and iPhone 8 than replaced a 6 that replaced a 5, 4 then 3G) and have never had these problems. Yeah, yeah, older devices uses the 30-pin connector.
(I just realized you said 12.9... At work, of the Pros, I have 4 12.9" Pros, but only one is 2nd gen and a refurb that was bought a several months ago. I have at least 2 10.5" Pros at work.)
I test software on iOS, so I have to have a lot. And, unfortunately, we have to keep some old devices around w/older iOS versions.

We can't just upgrade them all to the latest they support.
Are you putting constant stress on the port/connector/accessories attached to it?