In addition to all good suggestions above, here are some others:
If you want to keep the stamped steel fender brackets (I mean the ones from the hub to the fender, not the fork tab), get 2 sets- they are cheap at about any bicycle shop- online lowrider shops have these parts- epoxy or braze one bracket inside the other. Takes some work to get it right, but the resistance to cracking is more than doubled.
Better yet, go buy some 1/2" X 1/8" flat stock and make your own brackets. Bend and fit them until they are the right shape, then drill the holes where you need them. A bracket like that will not soon crack. You can use the same stock to make a tab to attach to the fork. I would prefer that over L brackets. They work fine, but are not as thick...and look like hardware store L brackets.
It would never hurt to use rubber washers where any of the brackets attach.
Finally, and perhaps the most draconian solution for having "fenders", but not dealing with this issue is to build simulated leaf springs- they are thick, strong and do the job of fenders without the worry. Look up some old motorcycle from the pre-war days. My front leaf (ornamental only) does not move at all- AT ALL- when I ride let alone vibrate. I am building the rear leaf this week (hopefully). Two problems with this- first, it only really looks right on a bike built to look like a pre-war model. Second- it is all self-fabricated. You won't find a kit anywhere and if you do not braze or weld, forget it.