Been wilting every 2-3 days and then recovers afte...

Increase P-K slightly. Your light is too close also. No fungus gnats can’t cause this.

I wonder why they wilting after 2-3 days, these seems like huge pots. Maybe you shoula add more water per watering up to the point that nearly no more water can be held by the soil. I can only tell for myself , my 11 Liter Pots get about 2 Liters every 4 days during that stage of flower, thats when first downer leaves start to hang down a little bit. Maybe you could start to defoliate some leaves who get no real chance to get any light. These are normally the first ones to show up nutrition deficiency and die first. That one middle leaf seems to have a calcium problem. If that should show up on other leaves, you propably want to add some cal/mag nutrition.