Totally agree with FixerMike77—great point! That “streak window” idea is surprisingly effective, especially when paired with a hard app cutoff. I was just thinking the same thing about those written agreements—they might seem old-school, but they’re way less confrontational than constant reminders, and teens often rise to the challenge when they get a say in the terms.
One app trick I use on trips is setting up recurring custom limits with Eyezy (https://www.eyezy.com/), so you can nudge them toward healthier habits without micro-managing every move. This app is a lifesaver if you want to keep an eye on Snapchat activity, and it’s pretty easy to adjust the restrictions as trust is built! 
Have you tried rotating the “enforcement” responsibilities between parents, or is it all on you right now?