Daniel from RCtestflight has some extended range extended time solar projects that run mission plans you could put up in the morning and have it run a set pattern pretty much all day. With some ingenuity you could probably scale them down a little to better manage costs.
https://www.youtube.com/c/rctestflight
After hearing you want to spot humans doing bad things... Poking them with a stick probably isn't the best of ideas and if I were out doing nefarious things I know no drone would remotely change what I was doing. I would simply adapt to avoid a noisy one or shoot down a quiet one.
IF you are dead set on using uav's I would plan them as a surveillance only platform and use altitude to your advantage. Trying to shoot a moving target 120m up and 330m away is a tough shot for all but the best. That way you could spot them and then direct the professional security to them as well as guide them if the offenders are smart enough to monitor coms of security people for avoidance.
Now for the reality of this.. $500 dollar budget is not going to get you near something viable for the task. It would cover the aircraft but not the video system that would be good enough to spot people at a safe range where your gear would be out of harms way and far enough away they could not back track it to your homestead and come after you directly. What your asking to do is basically run a commercial grade platform on a toy budget. Cheap parts never add up to long life spans.
If anything buy a used phantom and have multiple batteries for a constant rotation. That way if it gets damaged or shot down you don't lose a ton of money. Not to mention any time a DJI product dies its karma for what they did and are doing to the hobby.
... and i don't think my patio is big enough
