I would go and look at the phantom weapons again. I became pretty lost reading most of the stuff here but 20% additional assault added to each weapon? Pretty sure that is wrong. I mean you get an increase in stats of all the weapons that are the same as the phantom weapon and you get the second skill on the phantom weapon, when you have enough of required weapons in your grid. However, this is only for that weapon. So, a hammer has Carnage and it will give 30% assault. This second skill is just for the Phantom hammer and it won't pass onto any of your other weapons in the grid.
The phantom weapons do give an increase in base stats to the other weapons. So all hammers in a hammer grid (Including itself) will get an increase of 45% HP and 30% att. Other weapons in the grid won't see an increase in stats.
In short. I have no idea where you got the 20% for each weapon in the grid.
Also Thunder is good with hammer since they have good hammers to use. Due to tower it is easier to build grids around Kami weapons even for players that don't spend a lot and you can even buy the hammer for 2 ori in the shop.
I also wouldn't decide the soul I use on the phantom weapon in my grid and that includes the souls weapon as well. Usually you would go with Herc or Shingen for your souls. There will probably be times that the soul does vary like GO missions or Tower and perhaps other events in the future.