I think a 1/3 of a cup or less would be fine, I think your more than a little behind on the N.
Again the rose fert might work as well If you think that to much N might be an issue.
I would use the kelp and neem a TBS of each and see where it goes. I also sometimes forget container size. I am in 10 gallon plus so it would not be as concentrated that seems to mitigate some of the effects as well. That 2-6-4 would work if you have that concern.
I use Fat Flowers or build a bloom in late flower so I do not worry about the P now but that is just me.
Sorry a little choppy today. That Dab with the morning coffee might have been to much.