I have tried slower speeds but have now discovered that it’s not respecting upload_speed: it always seems to use 921600 regardless of what upload_speed is set to.
Investing that, I found Upload_speed not respected - #6 by neogene and a verbose upload does show the platformio.ini upload_speed for the esptool –baud parameter. But I don’t understand what the ‘related file’ neogene is referring to in his last post, so can’t resolve this.