I don't go over 900ppm in my own grow at the height of flowering.
During the first weeks of flowering this time around I was running around 600 to 700ppm and my plants did much better.
Avoid flushing. Instead, rinse. Coco will flush different elements at different rates and flushing the media will really destroy the Calcium buffer.
So, prepare a solution at about 250 to 400ppm and use that to flush with. This will still remove excess elements but without obliterating the element buffers as you enter flowering. I find that 30% runoff is a minimum when I see my runoff ppms more than 20% higher. Like, if I use 500ppm solution, and I get back 700ppm runoff, then that indicates to me a 40% increase in the runoff ppms. As such I would continue to rinse the media with my 500ppm solution until the runoff measured beneath 600ppm (within 20%). This will ensure that the media closely resembles your solution ratios without unused elements building up in the coco skewing the npk ratios.
Personally, I find Canna's Coco system runs just a little shy on Potassium. They have very good logic behind why they run low K levels but for cannabis specifically I think a little more potassium goes a long way in flowering. My suggestion to you is to drop your ppm levels to around 700 (1.4ec on a Hanna meter) and to try and balance your nutrient system with some pk. CannaZym is the only option for you in the Canna Family, so you may want to consider a simple bloom booster like Liquid KoolBloom at 0-10-10. Figure 7ml of A, 7ml of B, and 2ml of KoolBloom, and you're NPK ratios straighten out a little.
The first 3 weeks of flowering in Coco can be very tricky. It took me 3 or 4 grows before I really started to get a handle on things. Drop those ppm's and balance out the 5-4-3 Canna system to more of a 5-5-4 (I think 3-2-3 is ideal for this stage to be honest, but that will take a lot of work on your behalf to try and mix up) and try to keep them happy. A touch of Epsom Salt (about 20ppm) has done nothing but good in my own grow room.
The transition can be a little slow. Just give them time and care and everything will be cool.