Need a Sensor for Outdoor Gate, SAVE OUR DOGS!

Ok, so EPICALLY LONG story short… We have horrible neighbors who keep letting our dogs out and got our older dogs killed. I have a padlock on the gate and a camera pointing AT the gate, but there is so much traffic (foot and car) that the notifications are pointless, and he’s cut the lock off twice now. I’ve called the police, but he’s a rich doctor with friends in high places so no one really cares, and all you see is a hand and cutter, so there is no evidence that it is actually him.
Then add to that fact that the kids or I have left it open when moving trash, yard work, etc.

So I want a sensor that I can put on it that will alert us if the gate is open for more than 10 seconds or something. I have a place I can put it that would make if VERY difficult to disable/remove (ie, he would have to almost climb over it to do it and I’d get his ass on camera).
I have solid wifi by the gate, but no other devices so Zwave/Zigbee are out. It is outside, so it will need to be weatherproof of course. As for power, I am flexible. Battery powered would be good, but I can also use a USB power bank and run a USB cable to it.

The guy cut the lock and lett the dogs out again yesterday. Thankfully one of our puppies is NOT a runner at all and he didn’t leave the yard. The other puppy took off and THANK GOD she ran around and hung out in the front yard.

This spurred me to come up with something else and I thought I’d try to use Home Assistant.

This is based around homekit setups but the general setup is the same for the use case:

You just get a wifi based window and door sensor then put it in a waterproof enclosure then attach it to the gate you want to monitor then setup the automatons for your notifications.