Why did my cod turn into a hockey puck
A partial answer, to narrow down the possibilities.
I am pretty sure it's not overbaked. I have had overbaked fish, and while it does get dry, it doesn't get tough and rubbery. Rather, it starts falling apart, and the overall texture is quite chalky.
The &quot;puckness&quot; you describe does sound like it lost moisture in some ways, &quot;rubbery as to be unchewable&quot; and the comparison to cartilage are quite like the leathery quality of dried out fish.
The first culprit I am thinking of is freezer burn. If you didn't wrap the pieces perfectly during rethawing, and maybe even if you did, you may have gotten them freezer burned, or in this case I guess you could call it freeze-dried (although intentional freeze-drying would have used different conditions and would result in a different texture). This can readily account for the observed effect. The one detail that doesn't fit the hypothesis is your statement that the ends were better off than the middle - normally, the thinner a piece of fish, the more likely that it will dry out into that leathery state.
The second would be the rethawing itself. Normally it shouldn't have such drastic effects, but maybe you just had some kind of bad combination of conditions, or simply bad luck. I still find it less likely than the freezer burn explanation - while rethawing does cause a kind of dryness, it also contributes to mushiness, which is the opposite of the glued-together state you describe. But maybe it was the combination of freezer burn and rethawing such that each made the other worse.