How do I rig flip flop sandals correctly?

I’m working on an advanced rig for a character wearing flip flops. I’m trying to get the rig right and is almost perfect. The only flaw is that when you turn all the foot bones at 90°, the weight painting gets screwy. I think it may have something to do with the weight painting I did using vertex groups, but I don’t know what I’m doing wrong and I refuse to go without it.
Please help.

PS: I feel like the weight painting is too smooth and I wonder if it’s possible to sharpen it. Blender Guru did it before in a towel tutorial and someone did ask about it on another site (I forget which), but it said when I don’t have subdivision surface it’ll work tried it and it didn’t. I just want to know if anyone can help.

File: flip_flop_physics.blend

I’m using 2.83.1 by the way.

Also the bones (the ones that are away from the skeleton) on the outside don’t affect the sandal… I think.