There is no way to change the iron golem's damage output.

I tried giving it the strength status effect.

I tried giving it a netherite sword with /replaceitem entity @e [type=iron_golem] slot.weapon.mainhand 0 netherite_sword

I even changed it's damage from 7-21 to 10-30 using a behavior pack creator app, and it still had the same damage output.

It even stays the same regardless of the game difficulty.