You're actually underestimating.

Brilliant Light is 48 light. Light levels have been "doubled" 3 times since it first started.

When Sastasha is in Bonus (Blinding) it's 96 Light.

However, Feeble Light is 8 light. Ifrit NM gives Feeble. It takes 20-30 seconds to do. So in 6 minutes, you can do about 10-12 runs (with load screens). That's 80-96 Light all the time.

So unless Sastasha is in bonus or you have help running it, it's more efficient to just spam Ifrit