How does Fist of Suns work with my Commander?
Fist of Suns only replaces the mana cost. The mana cost is the cost written in the top-right corner of the card [CR 202.1]. You seem to think it's the total cost or the portion of the total cost payable using mana, but that's not the case.

Fist of Suns doesn't replace any additional costs or cost increases, so you still have to pay the Commander tax.

In game terms,


Fist of Suns provides an alternative cost [CR 117.9].
The Commander tax is an additional cost [CR 903.10].


total cost to cast the second time
= mana cost or alternative cost + additional cost and cost increases - cost reductions [CR 601.2e]
= alternative cost ({W}{U}{B}{R}{G}) + additional costs ({2})
= {W}{U}{B}{R}{G}{2}