You are describing my twins. I had preemie twins, both of whom have sensory processing disorder. They still have issues with it, but yes, their infancy was SO UNBELIEVABLY HARD!!!!!!
First, kudos to you for starting your baby in OT immediately. I am so glad I started them in OT very young. They made huge gains and it keep delays from compounding.
I found Harvey Karp's Happiest Baby on the Block invaluable in helping to soothe them. If you haven't already, I highly recommend you read his book and follow his advice.
Both twins screamed incessantly in the car. I covered their infant carseats with a lightweight baby blanket. It cut enough of the stimulation that they could ride without a massive, non-stop screamfest.
I also covered my son's pack-n-play for naps and bedtime with a lightweight sheet to cut the stimulation. Keep the lights off; dim lights helps a lot.
I kept blowdryers on an insane amount of time because the white noise really helped sooth.
Swaddling was my survival mechanism. I found a blanket with some stretch that was soft and pulled them into a very tight swaddle. They loved it.
Containment, being held firmly, with shooshing worked very well.
I would also try to intercede very early when they became upset or disregulated. They could NOT regulate or calm themselves. Catching dysregulation early and calming them was very helpful.
Reading with a very soothing voice helped.
Sucking, nursing helped my daughter immensely. I let them nurse on demand. Sucking is very organizing and being held closely with the warmth helped, too.
Firm touch feels good. Light touch can feel noxious.
Calming sounds, calming music -- watch your baby's reaction to see what music works for your baby -- helps.
I don't want to "lecture" or judge, but I will share that a good number of pediatricians refuse to vaccinate children with sensory issues. Their nervous system is considered compromised as is and vaccines an assault to their nervous system that they cannot handle. We did not vaccinate. You may wish to consider what vaccines, if any, you want to give going forward.
Also, I will forewarn you that in our experience and from discussions with OTs that I have learned that sensory issues can lead to or go hand-in-hand with other delays, especially fine-motor. They avoid experiences that would help their development. They dont' have good body awareness that helps them learn to hold a pen or control a pen in order to write. My daughter SCREAMED AND JUMPED DOWN IN RAMPANT TERROR. I just knew she had been stung by a bee, etc. No. The average grass of average height at the park was HITTING her. My sense is you are more needing survival tips at this point. I wish I had known that their avoidance behaviors could result in children dropping farther and farther behind because they hadn't developed the shoulder strength to use a pencil, the sense of their body necessary to write, etc., etc.
Its money well spent to find an OT very knowledgeable about sensory issues and aware how they impact continuing development.
I also will share that my son started out so overly sensitive to sounds, lights, etc., etc., that the OT was concerned he would be a child so impacted taking him to the grocery store would be nearly impossible. He also looked like a poster-child for attention deficit disorder. With lots of therapy, he has magnificient attention and I simply look crazy, at best, when I talk about where he started. He doesn't like his classmates talking loudly. He will tell them they are talking too loud and tell them to talk quieter. But he doesn't have a huge meltdown and become unable to function. Instead of wondering aimlessly unable to focus on any activiity or select an activity to do, he puts together 100 piece puzzles as a 3 year-old. They can make a lot of progress and do very well. And he still needs on-going weekly OT and weekly PT after very, very intensive therapy started when he was young. It's a sacrifice, to be sure, but the therapies have helped a great deal.
Feel free to contact me if I can help. I've so been there.. . . with two and as a single parent.
This is very disjointed, but I could tell you were desperate for some support and connection, not having had any responses yet.