I can't really comment on the bracket other than that I don't use it for the 1520 tank and I think I've actually lost the little extension wings that make the rollers sit higher to accommodate the 1520. When I use the machine with a 1510 or 1520 I just mount the tank onto the magnet and that'll hold firmly enough to get the job done. Probably doesn't help with the wobble, but since I don't experience that as a problem anyway, I never bothered to look into this any further.
If the wobble is extreme, I'd do as I suggested in my PS above and re-center the magnet onto the drive shaft to reduce the wobble. If that doesn't help, open op the machine and inspect the way the motor is mounted and fix any misalignment problems there. In my book it's fine if the tank wobbles very slightly, but it should by no means be extreme. I'd fix that instead of trying to work around it.
Btw, even with an extreme wobble I can't quite explain why you'd get uneven development. The tank rotates, so in the end, each part of the film(s) in there should be exposed to developer for about the same time, assuming the machine is mounted level and the drive shaft/motor is mounted level within the machine. Given your mention of an extreme wobble, I wonder if perhaps the motor is not mounted properly within the machine, so I'd start by having a look inside.
To fix the magnet, I think I used white silicone, but it could have been general purpose construction glue as well. Epoxy would work great here as well, and even acrylate may hold. I tend to use whatever I have on hand and that'll work, and if it doesn't, try a different approach. IDK, don't overthink it; this is a pretty straightforward mechanical piece of kit; it's not rocket science.