It's interesting that you didn't mention depression until the SWH addition. When I read the original post, my first thought was, maybe mom and dad have depression going on. And it turned up in the SWH.
Can you respect them for telling you frankly and early that they feel overwhelmed by the whole family there at once? They could have kept their mouths shut and hosted and been miserable for the sake of the appearance of a perfect family holiday. To what end? Because "other families do a lot more of that" than yours does, and you wish they would be less low-key because it's what you want?
Can you step outside your own emotion and personal disappointment about what "other grandparents" do at the holidays and re-frame this to accommodate mom and dad, who are after all the elders here? The way the post and SWH are written, the holiday disappointment is about your own blues at the folks not wanting to see everyone, all at once. Why not think about in terms of, "I get it, a group IS overstimulating and it would actually be more enjoyable to them if bro goes over alone to spend some time with them one day over the holidays; maybe bro's family goes for a short time at some point; maybe my family comes but does not stay in their house and keeps the visit short and does things with brother's family the rest of each day we're there," and so on..
If you want to see them, it may be time to adjust to what works best for them, and not what everyone seems to expect and what other families do or don't do. Bear in mind -- you still have them. My own parents are long gone, and my husband's dad just died. Your folks are still there and you say "I understand who my parents are" but you also seem to want them to change. Many of us would be glad to say, yeah, let's ramp back the crowd and keep it low-key. If their depression would be aggravated by stimulation, why not find a way to celebrate that is less likely to aggravate it?
Don't forget, too, to have some time alone with your own parents as their adult child, without your own kids and spouse there. They may really covet some time alone with you and alone with your brother. That's not a slam at the grandkids but a possible reflection of the fact they just might like to see their own children one on one. Ask.