The reason for using 74880 is that the data initially sent out during booting
is at that speed for ESP’s running at 26 MHz. If you are using an ESP at 40 MHz
then the initial BAUD rate is 115200, which is a standard rate.
The ESP does go into an auto-baud mode after booting but auto-baud is tricky and not
always reliable.
I think that my problem with bricking my OS this time is that the ESP booted
OK and then went into OS code which immediately panicked. This disrupted
the auto-baud and prevented the flash utility from grabbing the ESP and taking
control. By using 74880 the flash utility did not depend on the auto-baud
being completed successfully.
Not all USB to ASYNC adapters support arbitrary BAUD rated but the ones using
the CH340 chip do. However you must also have a device driver that supports
this mode, the standard Linux driver does not. The kernel module called ch341
does support the ch340 chip and arbitrary BAUD rates.