Anything that is going to reduce REM sleep is bad for your sleep quality and that's not going to be good for you.
Honestly, I would think the best way to deal with nightmares is therapy and addressing your underlying issues.
Nightmares, especially from something like PTSD I'd think, are a sign that there is something that needs to be addressed in your life and until you confront those things head on you will continue to have nightmares.
Most psychotherapists would say that nightmares are almost the unconscious mind's pain response in a way and most drugs will just mask the pain.
However, I would think if you actually have access that using something like MDMA or MDA while AWAKE would really help you confront those issues and help you have fewer nightmares if you use that time productively.
MDMA has shown to have massive therapeutical benefits for PTSD.
I would also think that using psychedelics like DMT, Mushrooms, Ibogaine, Ayahuasca, Mescaline, and to a lesser extent LSD or certain RC psychs could very much help you address your PTSD, but I'd think any of those should be combined with good therapy like CBT and or other modalities like neurofeedback, or even auto-hypnosis/hypnosis could help.
You might also strongly consider meditation like zen or mindfulness.
Ketamine therapy is more for depression, but I wonder if that might work also, but deciding to take something to repress REM just seems like a bad idea.
I mean eventually you will need good quality REM sleep and so you can only suppress those dreams for so long before it could really start to fuck with you in a bad way.
Also, though it might seem wacky, you might want to try to learn to lucid dream.
I have a very interesting book called "Lucid Dreams in 30 Days" which I've never actually followed all the way through, but it teaches you, well, to have lucid dreams in 30 days, lol.
You start by just writing your dreams down every day to starting to do cool stuff like tell yourself throughout your day that you are asleep and checking light switches and things like that to test your concsiousness and tell yourself you are waking up out loud before bed time, all with the intention to get better at controlling your consciousness so you can become aware when you are dreaming.
I'd think if you can learn to know you are sleeping in a dream you won't be freaking out in your nightmares.
They also make a type of machine that you can wear while sleeping that somehow teaches you to lucid dream by making sounds in your ears.
I really don't understand how it works but I believe it could and you can buy it for probably like $200 give or take.
I forget what it is called but an internet search should yield some results.