What would be the best flower booster for my situa...

You are using advanced nutrients for the basics nutrients so why not using their additives lines ? The extra PK bottle and the taste enhancer are enough additive or let's say that they are the most commonly used additives

Honestly any nute with good amounts of P and K in it. Brands are more or less meaningless. I honestly prefer dry nutes as you get more for your money. Check out bud explosion from Greenleaf nutrients for a cheap, no-frills bud booster. Its what I use along with megacrop from them. It works quite well and is very concentrated so a little goes a long way. I would buy it directly from Greenleaf nutrients as they have it the cheapest I've seen.

I'm using the AN nute line. You can keep things simple by just using the 3 part nutes. Or start adding in AN Big Bud product and finish up with their Overdrive product. I use a few other of their product like Sensi cal/mag which is important to use since your buds will use more of these micro elements during bloom. I personally have had better luck adding in Canna pk 13/14 starting last feeding of week 6 (2 ml per gallon) week 7 (2 feeding per week 3 ml per gallon) ending first feeding of week 8 (4 ml per gallon) then start your flush 10-14 days before harvest for a 70 day strain. Just keep an eye out for signs of nutrient burn on the tips of your leaves if you see yellow or brown tips lay off the nutrients and just use water. It would be unusual for you to use more than 1/2 strength nutes growing autos. Your buds will put on the most weight during the last 2-3 weeks. Also using a bloom light with a greater red spectrum will help with bud size your lights look a bit on the blue/purple side. Hope this helps👍

Just time. Buds will grow and fatten up. But big bud advance supplement is good.