So it seems my Thinkpad T14 AMD Gen 1 is bricked somehow. No idea how it happened. LEDs and fan are working but I don' t get anything on the screen (internal and external).
Disconnecting batteries and using the reset switch doesn't help.
It seems like these are the typical signs for a bricked BIOS and I'm thinking about trying to reflash the SPI NOR on the mainboard.
Oh dear, why do such things happen when warranty has expired...