Sub stats: ATK/Critical Mutiplier/Colossus. If u want switch element mods for better target-oriented, then u can come with a fire sub stat. Otherwise u'll have to put on fire mod for legacy's passive. If u decide to keep fire mod on then the 4th stat could be magazine/crit chance. Weak point is not recommended, base weak point modifier is too low, u can still gain damage bonus when hitting weak points from the innate weak point modifier.
If u have fire sub stat, then element mod is optional if u aren't dealing with end-game enemies. You can switch it to concentration support magazine, or u don't want to carry a ammo conversion tool weapon, then an ammo conversion mod or ammo maximum mod. Sharp Precision Shot is necessary, mental focus may only prove better on decsendants like gley. But still it requires longer time to warm up and it lack quite a mount of fire rate which is vital to DPS.
And finally a tip about slots. Many ult weapon have a innate rutile slot, and that sucks. If u want all element mods can be equiped in the future, then better not change that rutile slot to malachite cause it matches non-attribute instead of elements, if u do so u will need another slot to share some pols or polarize once more to achieve 4 element compatible builds.