Serial1 does not send data when RX is connected on boot.

The New interpreter error: BUFFER_FULL means that your ESP32 feeds the output to MDBT42Q
javascript interpreter console. as these are not javascript commands you should move console away.
But perhaps the

E.setConsole('Bluetooth',{force:true});
Serial1.setup(115200, { tx:D6, rx:D8 });

is too late and maybe will be always too late as the serial buffer will be filled with garbage sooner than those lines can run. Either there is a way to flush Serial1 somehow after you redirect console or maybe better is to simply use different pins.

Serial can be on any pins you chose, so if you actually don't plan to use Serial1 as Espruino console you may setup it on any other pins so it does not interfere with expected Espruino console input at boot time. So connect ESP32 to pins e.g. D11,D12 and then setup serial as Serial1.setup(115200, { tx:D11, rx:D12 });