The latest firmware automatically identifies the gpio chip number:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/gpio.cpp#L529
For RPi 5, I believe it will find gpiochip4. I am not sure where you changed gpiochip0 to gpiochip0, but it’s likely where you made the change is for other RPi versions, so it doesn’t matter.
The power issue isn’t just that the onboard 24VAC to 5VDC converter isn’t capable of delivering 5A; note that 5V*5A=25Watt, which means the 24VAC power adapter needs to output at lest 25Watt. Most common 24VAC power adapters is only rated for 0.5A up to 1A, so the power adapter itself isn’t capable of delivering 25 Watt to begin with.
Honestly I would always recommend using RPi 0 with OSPi — it’s the cheapest and it requires minimum power. RPi 5 is so power hungry that it’s approaching a mini PC which should use its own power supply. We could use a beefier voltage regulator but first, the issue is that the 24VAC adapter isn’t able to provide that much power to begin with; second, if RPi foundation releases RPi 6, that may require even higher power, and at this rate there is no way we can keep up. So if you must use RPi 5, the only viable way is to give it its own power supply.