Max temp limit in marlin

[Roxy]

If all you need are higher temperatures but can put up with the display being wrong then a resistive potential divider direct on the thermocouple will allow that to happen. The disadvantage would be that it would read 300 at 400 and 400 at 500 etc but it would still be accurate enough if you allowed for the constant temperature error

That's an interesting idea... You probably would need to adjust the PID control numbers too because that algorithm would be getting incorrect error numbers. But that would not be too tough to do.

Also... You want to avoid doing it, but you can turn off the Thermal Protection code if you need to. It is controlled by the

#define THERMAL_PROTECTION_HOTENDS in the Configuration.h file.

Also, another thing you can try is this. It may be you are having a hard time getting hotter and hotter. It may take more and more time to get a little bit hotter.
You can change these numbers in Configuration_adv.h:

#if ENABLED(THERMAL_PROTECTION_HOTENDS)
#define THERMAL_PROTECTION_PERIOD 60 // Seconds
#define THERMAL_PROTECTION_HYSTERESIS 1 // Degrees Celsius
#endif

And I just got reprimanded over in this thread: https://github.com/MarlinFirmware/Marlin/issues/2857 There is also a

#define WATCH_TEMP_PERIOD 16 // Seconds
#define WATCH_TEMP_INCREASE 4 // Degrees Celsius

On the heating up side, probably WATCH is more important.