Hi yogi1, thank you for the suggestion and the TYWE1S GPIO scheme.
So, I soldered as per these pictures:
And shorted GPIO and GND this way:
(yes, my soldering skills suck)
Then, I connected everything to my USB to Serial converter like this:
3v3 to VCC
GND and IO0 to GND
U0RX to TXD
U0TX to RXD
Unfortunately I am unable to obtain an IP address from Tasmotizer so the flash procedure cannot start:
I am really stuck. What am I doing wrong?