Not to be a dick but the question, although not a bad one, kind of misses the point. That said looks to be outdoor grow so yeah...
But my point is fat flowers are the result of many things along the entire process. Fattening them up at the end is sort of wishful thinking. To get fat flowers you need to really do a handful of correct things along the way to prepare it for a strong finish. Once you are in the home stretch they are pretty much on a predetermined path.
That said I find two things that make for exceptional yield. The first and most obvious is root ball size. Sorry not something you can do at the end but in general all things being done correctly the final weight is determined by root ball mass. If you have fewer branches they get bigger flowers but the yield is mainly limited basically by root ball size / uptake. There are many pruning and training techniques to help get to the max potential of said root ball. Not doing these things will reduce the ability to get to max potential.
The only thing I have found that actually boosts beyond the potential for the plant in perfect conditions is something to be applied early not late in bloom. Chitosan, which is the primary ingredient in Bud Factor X, makes the plant think it has an insect infestation which does a lot of things which are positive for the growth. Google benefits of chitosan on yield or something like that. It improves a lot of things but one is the plant tries to over produce flowers and resin to beat the insects. The plants goal is to breed and if it thinks it is going to be killed it will bloom hard and fast. Too much chitosan and the plant will hermi to try and beat the infestation. So I wouldn’t apply it after about weeks 4 or so of bloom and usually not more than twice. Once around flipping of the lights and once 3 to 4 weeks later after all of the original is gone.
Sorry bro... neither of those are things you can do at the end but maybe next year some drought stressing in veg and making chitosan in an EWC tea will help you out.
Only thing else I use would be high quality fulvic acid, which is a bit spendy, but will increas uptake. There are unfortunately lots of low quality Fulvic acid out there. You get what you pay for.
I go the super simple route and make complete soils with robust compost so my plants only need water. After years of farting around with chemical nutes. I find that all these things go away and it is just easier to get robust hearty yields that way using recycled living soils... so I stay away from the chemicals.
This clone was one week in soil before going into bloom and had one feeding of tea initially to liven up the microbes and only sugar water from them on. (So yeah not much in the way of roots but I was proving a point about clones to a buddy).
Rock hard nugs... but like someone said... I waited until ready. The real problem with outdoor in most places is you have to harvest too early. I can’t grow this strain to completion outside where I live it is just a waste.