Although I haven't personally used blumats, I can make some general recommendations for drip fees coco systems. You always want some runoff, and generally don't want to flush coco without your nutrient solution in the water. Stability and oxygenation is key. I would see if the blumats can give you 10-20 percent runoff per fertigation, 2-3 times per day, but I don't think that's the case so you may run into issues with salt build up. In any drip feed system it's good to hand water a few times a week to make sure complete saturation and proper runoff is occuring as well.
If the blumats can't allow for this with their gravity feeding system, I would try a pump and timer system to allow for proper runoff of fertigation and distribution in throughout the coco. Or hand water 1-2 times a day. You could also run the EC a little lower, I find 1.2 works very well at your growth stage, but you may still get salt build up at the lower EC without the fertigations evenly flushing through the medium.
Other than that you're doing great, and good luck with the grow! 👍