The hard part is that there is no effective way to regulate anything in the supply chain involved except for the explosives themselves. Everything else is super commoditized at this point and, other than the props, very multi-purpose. The first significant hexcopter I built used a BeagleBone Blue for processing, generic ESCs and BLDCs for the motors, and an aluminum frame that I cut out of aluminum tubes from Home Depot. Max takeoff weight was 55lb, because that’s the heaviest it could legally take off with. This was 7 years ago.
one thing in societies favor though - sufficiently unstable lunatics tend to self delete themselves in various ways by being unstable lunatics. few tend to be in the “sweet” spot of dangerous lunatics who are stable and focused enough to follow through successfully with a dangerous plan. thankfully.
For example - most people who could synthesize multi-kilo quantities of TATP without blowing themselves up and successfully build a DIY drone to carry it have better and more productive things to do with their lives. at least in the west.
Even without vulnerabilities like that, something like https://comma.ai/openpilot could very likely be used in the same way ArduPilot was used in the recent Ukrainian drone attacks.
a van is just a bigger, more inherently stable drone.
Plus, you got a cool and potentially lucrative hobby, designing exterminator machines. Why bother with children at that point?
There are much, much better targets to be had.
Your point on the dwindling barrier to implementation stands.
the premise is that the person doing it is very mentally ill. the question, "why would they do that when they could do something else that makes more sense?", doesn't make a lot of sense itself under the premise.
We need to minimise the damage they can cause, and that means preventing them from using slaughterbots.
America insists on making sure that guns are universally available so that school shootings can still happen. Doesn't register. The death toll seems to be politically acceptable.
Nice.