So after completing IST-KUL-SYD 18hr journey on TK's newest A350 bird (LWD) my thoughts on the best seats.
If you are tall like me (195cm), you can't beat the extra leg room offered in Row 1 bulkheads, both window and centre seats. Simply put, the "box" into which your feet go when the seat is reclined into bed mode is at least 10cm longer and slightly wider than any other row of the 32 seat configured A350.
Row 1 centre seats (1 E/F) also provide more space to enter and exit the seats from the aisle.
I travelled in 1K but as the final leg of my flight was basically empty I tested seats 2K, 5K, 7F, and 8K all in bed mode, and none of these seats allowed me to fully recline on my back. Unfortunately the seat width especially shoulder area still remains an uncomfortable issue.

BUT, be aware you will experience significant noise from the crew working the front galley if you select the bulkhead seats. So as a tall passenger decide yourself whether you prerfer extra comfort, or extra quiet

Safe travels!