What about something simple, like white noise? We use that in my son's room, and he can't hear hardly anything when just a regular vornado fan is running in there. It's eerie. Then of course you have the white noise of the fan, which is annoying, but it might be interesting to see howthat combines with your earplugs, which are great at blocking noise of that sort I presume.
It might be totally unworkable, but it's also pretty darn cheap (possibly free, if you already have a loud fan) to try.
The other thing that occurs to me is cheap DIY door soundproofing. We hang blackout curtains on my son's windows so that he can nap, but we made the discovery that it also really blocks incoming sound from outside. Not completely, but it really helps a surprising bit.
This, again, is free: if you have a heavy quilt or blanket, you could experiment with it on both sides of the door and see if anything helps.
Maybe you already tried things like that, but it seems to me that you aren't in need of perfect sound damping: you need to reduce the incoming noise from your primary vectors of incoming sound, and then your existing solution will probably take care of whatever slips through. But instead of stacking more earmuffs on or whatever, you're blocking something better, aka general room noise in one fashion or another.
Another thig you might look into is a standalone active noise cancellation technology. Not earphones; this is something that sits in your room, plugged into the wall. My dad has been around those before, years back, and said they were amazing. They could basicLly eat all the conversation from someone right in front of you. But these were big, commercial, presumably very expensive devices. It was also a decade ago. So I have no idea what is on the market for that sort of thing, if it works, or what it costs. But the ethnology itself is very real and not bunk; even if there is likely a lot of snakewater oil being sold under that market name. I have no idea if that's cost feasible, and it mitt be overkill, but it would also be something you could bring with you to future houses or on trips, etc.
May not be helpful at all, but I know very little about this. I dabbled with noise cancellation headphones, but the active ones give me a headache despite working kind of well. So I just got used to playing music through my normal speakers, and tune out the other noise. That doesn't work for everyone, though, and there are definitely times when it doesnt work for me either.