I doubt that I can add much beyond what was discussed prior.
Melatonin did help me eventually and I think the concept of using every available means to be more awake during the day might be helpful at reducing movements and treating insomnia.
I haven't tried any of the newer range of memory foam topped mattresses. It's possible that they might be more effective at absorbing vibrations or movements. Perhaps you could try out some at your local bedding outlets.
Hopefully others can offer more useful suggestions.
Neither of us have PLMD, but we made the switch from a Queen size bed to a King size. It works quite well. However, a king size deep mattress is a monster to handle. I thought at one point we were going to have to return it as our bedroom is on the second floor, and the stairway was not cooperative. If I had it to do over again I would have gone with two Twin XL mattresses on a King size bed frame. You can cover both mattresses with one King size sheet set. Some hotels/resorts do that to allow flexibility on how they set the room up. My other suggestion is to use a slat type support system with a rigid center support instead of going with a box spring. Last we have found out that a bed frame that encloses the mattress to some degree is a real pain when it comes to make it. It is hard to get the bottom fitted sheet down around the corners and hooked so it will not come up during the night. If I had that part to do over again I would go with a platform style bed frame that has the mattress sitting on top of it. Trying to lift one of those 18" deep King size mattresses while slipping the sheet under the corner is not an easy task. Incredible how heavy they are.
I also just switched from a queen to a king and from a 'regular' mattress to foam. With both changes I noticed a big reduction in how much tossing and turning from my spouse (and doggy) I notice at night. I honestly barely notice they are in bed anymore!
We got around the monstrosity of the king issue by ordering the mattress online. It arrived in a box, which was horribly heavy, but at least we were able to handle it and get it up the stairs of our very small house. My only fear is when we move :). The idea of getting two twins is interesting!
All this is with the caveat that neither my husband nor I have PLMD.
Best of luck!
I can relate to the struggles of having a partner with PLMD, it can definitely impact your sleep quality. While getting separate beds is an option, I understand why you don't want to go that route. Have you considered investing in a mattress that is designed to reduce motion transfer? Some brands make mattresses with special foam layers that are specifically designed to reduce movement and provide better sleep for both partners. Another solution you could try is using a white noise machine or a sleep aid device like the Dodow Sleep Device. Does dodow work for sleep? Yes, it does. This device uses a rhythmic light system to help you fall asleep faster.