Can't figure out how to change the proficiency bonus on my homebrew weapon

I'm trying to make a joke weapon, a Morninstar you smack 3 times called "Sweet Caroline". Each hit is supposed to do 1d8+2 damage, but every time I equip it to my barbarian character it becomes 1d8+8, how do I change the fixed bonus so it only ever deals +2 regardless of stats?

That is far enough outside the normal rules of how weapons work that it's not possible with the current homebrew system.

You can customize the attack on the character sheet to add a negative modifier to cancel it out.

Didn't know you could modify it in the character sheet as well! Now I got it down to a reasonable damage, thanks!