Mistwell
Crusty Old Meatwad
Did you ever talk to your DM about these concerns? It sounds like that might have helped.
Yes of course. He's a reasonable person. It's his fundamental view that his job is to challenge the players, wherever they are at. So odds are if I just stopped focusing on AC and focused on something else, he'd change his focus as well to meet that new specialty. It's too late for this campaign though to re-do my PC.
To address this on my side, as soon as we end this campaign (which is probably in about a month, after years in this same adventure) I plan to start a new PC that's more evenly spread across many different areas of the game and see how it goes. I won't be the guy tanking. And if we have no tank, let's see how the DM adapts to that.