I have tried the above Optiboot bootloader/s with mixed results.
The flasher kind of works , but none of the receiving (Rx) bootloaders .
I found a nice OTA project by Ben Amsellam on bitbucket and one of the rx bootloaders worked to the point where it began to write to the target then the dreaded “out of sync” msg came on. It was the bootloader with the 4 second wdt. It was also the only one which showed some activity on pin 13 (clock) on the oscilloscope.