I'm on Android. I will try the things you said. Thank you very much!Are you Android or iOS? Most of my list is geared towards turning off all the various android auto updates and backups. As well as some specific apps I use for photo cloud storages (DSPhoto).
I also set up the DCL wifi as "metered" under android and turn on the data saver. Once you do that you can restrict which apps are allowed to use data in the background.