I think Martin is going to take the 49ers Larry Allen trajectory and just become an immobile phone booth guard. He has higher football IQ and better technique than a lot of position coaches, and like old boxers, power is the last thing to go. He's just going to get slower and stiffer as injuries catch up to him.
I could see him playing RG at a starting level into his late 30s. Guard's not a "matchup" position where you have to handle another elite athlete 1 on 1. It's a "teamwork" position that's more about coordinating assignments with your unit and getting yours right, and Martin doesn't screw up. He'll age well. He won't be the best guard in the league, but he'll be a good guard for a while longer.
Like others have said, Martin also seems like a guy who would rather retire a year too early than a year too late. So much guess is he walks away before his tank fully runs out of gas.
Kind of like Frederick... he could play center at a functional level after his recovery from Gullein-Barre, but that top end strength was gone. He figured he'd pocket his money and call it a day rather than take more damage playing, by his standards, disappointing football.
Martin won't walk after this year, not after fighting Jerry for extra guarantees, but anything after the 2024 season concludes is a possibility IMO.