Please refer to the data sheet.
The Nextion Enhanced 5.0" will be limited to 250 nits (250 candela)
I would have to assume that it would be much less than adequate in sunlight conditions
thus making it useless or mostly useless for such applications that rely on info being seen
Is it possible to replace the backlight LEDs for brighter ones?
Anything is always possible - however, probable and successfully - no
- and still keep your warranties ... ummm no.
You must pick the right components and tools for the job
- even if it means this one might not be the right one for this job.
In a vehicle where it is important that the displayed info being seen is almost vital - I'd go with safety.
Itead most certainly would not want to see anyone injured from incorrect usage of their displays.