Deploy Javascript files to a Puck from native code

There's no better option than the BLE UART if you want to do it wirelessly. You could implement your own characteristics, but it'd be just as slow - if not more so.

The issues with writing while Puck.js is transmitting could just be a buffer overflow - if it's waiting to send data while also receiving stuff.

When you're writing code I'd definitely try doing what the IDE does though, which is to send character code 16 at the start of line for each new declaration (which disables echo for that line). You can get the espruino NPM package (the CLI tools) to output the exact characters it would have sent to a file, which might help you there.

Otherwise you can just send "echo(0)\n" before everything to totally turn echo off for all commands.

I'm afraid I'm not up to speed on the Android BLE APIs so I don't know if there's a faster way to do BLE - but I guess there must be if Web Bluetooth can do it. It might be worth looking at the sourcecode for the Nordic UART app and seeing if they do something different?