I think you are not understanding how this works.
The DC car charger input you are currently using is limited to 8A - doesn’t matter how much power you supply, the EcoFlow will limit input to 8A.
When it comes to electricity, it is easier to compare wattage rather than amperage. Watts = amps x voltage. Just increasing amperage alone won’t make anything work out better - in fact, it will probably damage the existing wiring. Likewise, just increasing the voltage won’t help, either, without some method of converting that voltage back to whatever the EcoFlow wants.
This means at 12 volts, the max the EcoFlow will accept (not what you supply) at the car charging port is 96 watts. If you did what the video suggests, without knowing exactly how the EcoFlow BMS behaves, the maximum amount of continuous power the EcoFlow will accept is 896W (800w max solar input + 96W max car input). That is a pretty good bump and maybe all you need.
This is exactly what I mean.
The EcoFlow has only one DC input. Car or Solar, automatically detected and limited to 8A.
When imitating Solar charge by boosting the voltage , let's say 24V, you can get a 100% improvement in the charging wattage. Still limited but twice as fast. This is all I need.
If you want to charge it was quickly as possible, your best bet is cobbling together some sort of AC power source from the alternator (alternator -> batteries -> inverter -> EcoFlow). Even including conversion losses, that would probably still be a decent amount of input wattage when compared to either DC alone or DC + solar.
DC to AC to DC is too much energy loss for my humble opinion.
It is surprising to me how popular these things are given their substantial limitations and cost.