Sorry, I clearly misunderstood - so the problem isn’t the tripwire implementation but rather the high current that you need to switch. My first thought was (as someone else suggested) to use a relay, and a quick web search brought me to this mention of “Darlington” transistors: https://itp.nyu.edu/physcomp/lessons/electronics/transistors-relays-and-controlling-high-current-loads/

I couldn’t imagine the transistor flipping when it’s base is actually grounded. Did you try an NPN transistor? I am not familiar with FETs, maybe they behave weirdly here?

I was never excellent at analogue electronics but your very first diagram looks perfectly fine, with the tripwire connecting the transistor base to ground. Preferably with a large resistor to avoid draining the battery quickly. A grounded base should effectively close the collector-emitter path.