My question is if dealing with the reversed polarity could damage the GPIO pin or even the whole board. Basically Yes, If you look at the schematic for the board 5v goes through a 2k resistor to one end the led and the other goes to the the charge controller (pin 1). To be safe you could either A) ...

Just use a transistor or fet to power your PIR, and control it though a GPIO pin.

Maybe something like this? Trigger it with one of your GPIO. https://www.amazon.com/HiLetgo-Trigger-Adjustable-Control-Conduction/dp/B0832MW9L4/ref=sr_1_6?crid=2AQX3POCR5PLS&keywords=digitally+controlled+power+relay+with+time+delay&qid=1707712926&sprefix=digitally+controlled+power+relay+with+time+de...

remove the ESP_LOGI's from the ISR handler and see what happens.

need to supply a little more to the story.
What voltage are you supplying the L7805CV ?
Did you put filter caps on the input and output, and if so what are they.

what has worked for me is when I create a new project I typically get a Triangle in the lower right of the IDE.
Click on this, then you will get a popup on top, select the first one, and that fixes the issue (for me anyways)

It's not specifically geared towards your specific flavor of the ESP32, but it would not matter much at all.
https://learnesp32.com/videos/course-in ... troduction

Can you post your code?

However, the actual manual switch on my fireplace runs with very little voltage. About 0.2v.

Something does not sound correct there. Maybe its AC and your reading with a DC meter ?

Best bet is to use a relay.