Applying a weak 500mA USB power to the USB connector is no problem. The onboard 5V voltage regulator gets reverse current, but that is okay.
If you apply suddenly a very strong 5V to the USB connector, then the current peak can damage the board.
Using the 5V pin is more dangerous, because there is often no polyfuse and current can go into the computer (from the Arduino board via the USB cable into the computer). It depends on the board and the circuit. But it is mostly the same as with the USB power. If you apply a weak 5V to the 5V pin, then the voltage regulator can deal with the reverse current and the computer will probably not get damaged by the current into the USB bus.
Your Nano Every has a diode for the USB 5V. So the computer is safe, whatever you do.
I'm reading here that sterretje uses a 5V to 9V converter and applies 9V to the Arduino board: Smoke when powering Arduino UNO with 12v 7a - #2 by sterretje
I would go for 7.5V. When I use the barrel jack or VIN, then I use 7.5V.