I think the story goes that the later 5 spoke alloys are stud centric, this means that it's only held central by the wheel nuts, not the hole in the middle of the wheel. As this hole is a cast finish it may not be central to the rotation of the wheel, so most cone balancers which use this may not balance the wheel accurately. For the most part it's close enough, but to be 100% accurate you need to find a balancer that can accommodate stub balancing.
Yes it's possible for the tyre to slip at low pressures, particular if it's wet and you have a nice torque supply behind it such as in 4L.
Fatter and heavier tyres just highlight any imbalance more.
I would start with a simple balance again on the front just to eliminate movement or wear on the tyres.
After that ???? Find a stud balancer, balance beads as Riccy suggests.
I have the same 5 spoke and have not had any issues with cone balancing, but maybe I am just lucky or the standard tyre size helps.
Good luck.
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