We were just talking about this the other day. @karltinsly wrote an arduino gcode sender a while ago and I just had to look up how he manages the serial connection.
Grbl has a buffer of a few lines of gcode. It will send an ‘ok’ back as soon as it has your command in the buffer. So sending the next command after you see an ok should be good enough to keep smooth operation.
The endstop nose is strange. It sounds like you need a pullup or pulldown resistor to keep the pin from floating. But we also use NC endstops in most configurations, which reduces noise by constantly sending a small amount of current.