The best solution of your problem is using a High discharge Li-po battery cluster(12V) with a 5V-10A BEC. In this way you can get the power to run two pi's efficiently and without any throttle. But you need a powerful battery if you want to use it some more time without charging it now and then.
A single battery doesn't have that much discharge rate.
I've successfully powered a Pi 3B+ using a cheap 12V 3A wall wart from aliexpress and a cheap buck converter from aliexpress. The 12V 3A provides for well above 3A when dialed down to 5.1V through the buck converter.
The most difficult part is soldering wire on to a USB plug.
A raspberry pi need 5 volts to operate. So use a voltage regulator . It is best to use the official PSU though. But a 2 A power source is good. You can use the BEC(that is a very good idea). But keep the voltage regulation as low as possible(Assuming you are using an inverter)
7805 can not give you sufficient amount of current. According to the datasheet it can give you maximum of 1.5A(You need a heat sink and cooler for that).
You have to use around 4 7805 IC for this operation, and also a filter system.
There is also some other risks, if you are powering your sensors or module from pi then there is a chance of over current ...
As pointed out earlier the problem is not with the power supply. 1 A is enough to run a RPi.
Here are some ways to revive a RPi
Try the pin method
sometimes the SD-card reader on the pi gets deformed over time, so applying some pressure on the SD-card with a pin or folded paper could work, also cleaning it with a brush works sometimes.
refer to this for ...