I'm stupid and can't tell why my new / "better" weapon does less damage

Maybe the additional 6% phys damage converted to fire?

The popular guess: does you have inc crit, dmg or anything at full hp? Because 3str diff probably put you at not full hp so you lose those bonuses

Did something else unequip, because of the loss of 3 strength?

Could you post your build in more detail? Like a POB

POE2 has sooo many things that can have effects on each other it is hard to "debug" this without more details.

Example: I recall there are support gems that have modifiers to damage based on how long attack time of a skill is where perhaps faster attack time can create less damage.

In any case, what does the maul skill show as damage when you take out all of the support gems? still the discrepancy? if not, find out which support gem is at adds with something by adding them one by one and swapping weapons each time.

Also turn off all aura effects or buffs and check that baseline too.

Losing 10% reduced attribute requirements makes any of the gear of gems inactive?

Thanks for all the help and suggestions!
Apparently, I am indeed very stupid
It really was the 3 strength that made the difference
I don't know why though, I checked all my Equip and my support Gems but I can't find anything that need these 3 points to activate...

In path of building you could edit the weapon stats down from 86% conversion to 80 and see how that affects damage. Import your character and then edit. Help confirm what the issue is.

It is the Elemental Armament II supporting Maul got disabled for lack of Str.

I play the same build and I think you can remove cold attument. The reason the guide use it is to freeze enemy faster.

However as long as you have the "add cold damge" stat on both of your rings the additional cold damage is not needed. You will be better of with close combat or something linke that

I’m with you! Unless the strength makes a difference on another item, we might have to see a PoB to determine