How can I take on-device screenshots of "secure" apps?
As your device is rooted, you could use the Xposed module DisableFlagSecure:


  An Xposed Framework module that disables FLAG_SECURE on windows system-wide. This lets you take screenshots and do screen capture in apps that normally won't let you. (That said, you probably don't want to leave this on permanently).


You can of course also find it in the Xposed Repo.