You do not control when chars arrive. If you do not pull them they are lost! So if you say 64 buffer (btw. arduino implementation has only 63 chars working if you say 64 byte). We also do command by command but allow input buffer to fill up for faster communication. Set host to ping pong if it should wait for "ok" and not fill up buffers.
For line by line disable temperature checks in host and send them in manual control line by line. There is no debug for this available.