Just played with a friend, and he says King's court first player that gives a least 7 gold advantage (2 for tax exemption and 5 for metal ) is maybe too good, considering metal is a rare building material. I think maybe changing it to metal only (no tax exemption) as a variant rule is still a good choice?
You should leave the rule as is and not try to change it for a few reasons:
1. The spaces are not supposed to be all as good as each other.
2. The key spaces are generally the ones for the craftsmen and not the metal space.
3. The first pawns out of the bag can pay as much (or almost as much) as the benefit you get from taking the metal and tax relief.