Well the flash procedure with two wires from GPIO pins was a bit cumbersome, but I got success reported . Anyway, I just succeeded by following the instructions on SEEED reTerminal display stm32 stuck in boot mode
INCLUDING the part where you have to remove the battery (!). Before that it didn’t work. So in addition to all other advice that is filling these blog pages and for which I am grateful, that is apparently what you have to do:
Remove and replace the battery.