I'll let you get it up to 10L/2 without any increase in attune cost. 11L is Grand Daiklave territory - so at 11L and beyond, increase attune to 7m, cap out at 13L. Can't let you increase the Overwhelming value beyond 3 (that is definitely Grand Daiklave territory) - so a total of 13L/3, final attunement cost of 8m.
Also, I fixed the Thunderbolt Shield stats for you (they're also in errata, following the 2.5 change of shields being high-defense, low-damage weapons).