this is such a gamer post.
sitting with good posture is not good for long sessions because you are the opposite of relaxed. if you sit back, slouch, and relax into your chair, you'll take strain off your body. That's why I don't really agree with these "ergo" people that suggest you need to sit upright anytime you're in a chair. Just get up and stretch every now and then, and slouch away and you'll be fine.
I have a Lay-Z-Boy big and tall chair, which is way oversized for me. I have a foot rest that lets me lean back and this basically keeps all pressure off my spine and I don't have a problem sitting in whatever way is comfortable.
Think for a minute, does sitting in a car for extended periods of time cause you the same discomfort? Are you able to take a one hour drive with your seat leaned back a little? Apply that same logic to sitting in a chair. If you sit in a relaxed position, you'll play more relaxed. hope this made sense
Haven't personally tried it, but both steam and Windows like to recognize ps4 controllers as xbox ones. Try downloading a copy of DS4Windows to use with the controller, it's much better at registering properly.