Ok, I've solved it with this sequence.

1. Erase and then flash UIFlow with M5Burner.
2. Disconnect USB from PC
3. Press PWR Button 6 sec to turn stick off.
4. Connect M5Stick to PC again

Ok, I've solved it with this sequence.

1. Erase and then flash UIFlow with M5Burner.
2. Disconnect USB from PC
3. Press PWR Button 6 sec to turn stick off.
4. Connect M5Stick to PC again

Same stick, same problem. I was trying to flash stickwatch project (https://github.com/sysdl132/m5stack-stickwatch) through platformio with settings indicated by the author:

[env:m5stack-core-esp32]
platform = espressif32
board = m5stack-core-esp32
framework = arduino
board_build.flash_mode = qio
board_build.f_flash = 80000000L
board_build.partitions = no_ota.csv
upload_speed = 750000

I've tried with the BAT + GND solution but with no success :(
Tried erasing and loading m5stick_factorytest firmware.
Tried erasing and loading uiflow-v1.4.3 firmware with m5burner:

esptool.py v2.5.0
Serial port COM4
Connecting....
Detecting chip type... ESP32
Chip is ESP32-PICO-D4 (revision 1)
Features: WiFi, BT, Dual Core, 240MHz, Embedded Flash, VRef calibration in efuse
MAC: d8:a0:1d:50:51:d8
Uploading stub...
Running stub...
Stub running...
Erasing flash (this may take a while)...
Chip erase completed successfully in 2.1s
Hard resetting via RTS pin...
Finished

Command:
--chip esp32 --port COM4 --baud 750000 --before default_reset --after no_reset write_flash -z --flash_mode dio --flash_freq 80m --flash_size detect 0x10000 application_0x10000.bin 0x1000 bootloader_0x1000.bin 0x200000 fatfsImg_0x200000.img 0x8000 partitions_0x8000.bin 0xf000 phy_init_data_0xf000.bin
===================================================****
esptool.py v2.5.0
Serial port COM4
Connecting....
Chip is ESP32-PICO-D4 (revision 1)
Features: WiFi, BT, Dual Core, 240MHz, Embedded Flash, VRef calibration in efuse
MAC: d8:a0:1d:50:51:d8
Uploading stub...
Running stub...
Stub running...
Changing baud rate to 750000
Changed.
Configuring flash size...
Auto-detected Flash size: 4MB
Compressed 1965600 bytes to 1378389...

Writing at 0x00010000... (1 %)
...
Writing at 0x0021c000... (100 %)
Wrote 2097152 bytes (125780 compressed) at 0x00200000 in 2.5 seconds (effective 6809.0 kbit/s)...
Hash of data verified.
Compressed 3072 bytes to 122...

Writing at 0x00008000... (100 %)
Wrote 3072 bytes (122 compressed) at 0x00008000 in 0.0 seconds (effective 1540.8 kbit/s)...
Hash of data verified.
Compressed 144 bytes to 69...

Writing at 0x0000f000... (100 %)
Wrote 144 bytes (69 compressed) at 0x0000f000 in 0.0 seconds (effective 72.2 kbit/s)...
Hash of data verified.

Leaving...
Staying in bootloader.

Stick is working, but blank screen.
Maybe I've messed up partition table or something... I've not connected anything more than the USB-C cable to the computer and shorted the GND-BAT pins to try to solve the problem, so couldn't be a hardware problem...