I don’t have much experience with autos, but I’m gonna guess 5 gallons will be just fine. I think you have a really good base so far. I have not seen the consistency of Bu’s potting soil. Adding a little coco would not be a bad idea imo. 40% might be a bit much though. Hard to say without seeing it. I would definitely put it in a tub and mix it all together. If you wanted to just top dress with EWC that would be fine also. Let me know what you find at the grow store. I might be able to help source amendments. You don’t need much. 
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