So, any solution has to take into account this power curve

OP's solution does seem to address by limiting the amount of people one can vote for. Would be simple to work around, but would still limit the power of the biggest holders.

2 accounts with 10 mil sp voting for 20 witnesses seems easier to fix then 1 account with 20 mil sp voting for 20 witnesses.

Could also add another type of witness, call them community witnesses or something. Allow them to only be voted on by the 20%. 4 of these positions so the 20% could prevent a hard fork seems like a decent option.

Though both of these couod be terrible really, idk