Thauma buffs currently just increase your stats. Agni’s is a damage multiplier so yes they work together.
While the swell ktos buffs are nice. The amount of attack obtained from int is fairly low since its basicaly just x0.28 int for the combined boost from both arms. Thaumaturges attack buffs were underwhelming even back with only r7, and they will still remain underwhelming. Mainly due to fact that gems/hats/gear got massively stronger and transcendence levels will be easier to obtain with the recent ktos patchs.
Currently right now I get +732 matk with right hand wielding dagger, left hand, and swell brain.
As a level 321 full int my buffs will give 1087 matk with the new changes.
1087x3.8x1.8= 7,435
The Effect it would have on agnis + fireball 80% attributed would be an additional 7435 damage per hit as long as your already bypassed the mobs mdef with your non thaum buff stats.