hello,dan here,does trimming the corners of your reed help with sinking back D?i have learn to let off on the pressure when i hit back D,without drones changing much,but would like more stable thumb note.i affraid to trim any more,for fear of getting too sharp.i have 5 reeds thati have made,all play well,and pitch threw both octaves,back D is the only issue. THANKS, yes or no? HA!
Yes & No 
Sinking back D is usually due to the reed being too thin in the centre of the lips. Try making a tougher reed that needs to be played in over a few weeks, instead of trying to make an easy playing reed right from the get-go. Trimming the corners won’t really help much for sinking back D.
djm
Tip: to become a seasoned player, rub yourself in linseed oil twice a day ad infinitum…
I make no pretence of being a seasoned played or reedmaker but I’ve heard that leaks at the sides of the reed can cause the back D to sink. Solution? Lightly coating the sides of the reed with various substances has been suggested - bees wax, nail varnish, glue, etc.
Contrary to all wisdom handed down my back D becomes troublesome when the reed’s too open (Geoff Wooff had some theory about the reed having to too slow a rate of vibration when played to open to play back D properly but never mind that).
Experimenting to solve the problem is your best option.
Agreed. Sometimes closing the reed a bit works for me. Less pressure overall is required to play. Sometimes combined with tiny amounts of bridal (height) adjustment… however this may affect (cause) tuning issues.