So 0x0000 is a sign that the unit is patched and won’t accept a payload. Unless you fit a 3rd party glitching setup that you actually fit to the CPU, you’re not going to get around that.
The Fuel IC is pretty critical and provides battery consumption rate, battery level information. It is checked very early in the boot process and if the boot processor is unhappy, then it will halt execution before screen is enabled or logos are displayed (and also prior to fast-charging commencing - current >1A), so it’s an area to focus on. If you are unsure about your fit, then I’d focus on getting comfortable that it’s correctly installed (check to see if it isn’t lob-sided and there aren’t any bridged balls etc.)
Post a picture if you can to help us see what you are looking at.
The advantage of using an RCM payload is Hekate for example will still boot to the text mode UI regardless of the fuel cell status, so you can at least get a sense of the issues.