@Lastic, it might be worth giving the firmware attached a try.
In developing it I think I recall it going through a period of instability on some machines which eventually passed.
Its compatible with the existing firmware, I'd mostly split up the sources for easier understanding.
The extra two roms are demos of using code on the STM32F4 to benefit from sharing the cart area memory.
Just reflashed the STM32 with your attached firmware but no luck, 130XE goes straight to BASIC.
I'm going to recheck my soldering , it's the only thing I haven't done yet.