I would encourage you to try langbeinite too. I.e., boost K, Mg & S (leave P out of it) in late flower.
Example:
2 Tbps AK Kelp = 84-92 ppm. (K=40-48ppm; N=10; Mg = 5; Ca=15 [a good ratio Ca:Mg for veg]; S=2. ).
1g langbeinite = 139-149 ppm. (K=48-58ppm; Mg=29; S=58).
Cut your base nutes a little to accommodate 100ppm; alternate between those two. Or, cut the base nutes enough for 200ppm; feed the above together.
I've used langbeinite randomly in flower. Definitely the kelp makes a difference in late flower. But, boosting K, Mg & S too should help. (I think it's the growth hormones that make kelp good. But, others say K & Mg make a difference. That would be a way to get both. As long as you cut the base nutrients it wont fry the plants (like typical PK boosters can).