I’m somewhat confused about this. A little slack is actually fine, and better for the lifespan of the drivetrain. It’s also pretty traditional for track bikes to have slightly slack chains. But it’s hard for me to imagine how your chain could go so slack that it would be at risk of coming off, unless your chain line was really bad. Of course if you’re riding fixed and doing skids, you never want any slack in your chain. So if you did that, stop.
It just shouldn’t slip that much, especially if you already have the nuts tightened down to some degree before you set the tension. Sorry if this seems like a stupid question, but are you using proper track nuts? Because if you believe your technique is fine, maybe one or both of your nuts is slipping because they aren’t holding? Or maybe the threads on your axle are bad? The 6ku is a pretty cheap bike, so it’s not out of the realm of possibility that your equipment isn’t any good. I don’t say that in a condescending way, but just to point out that it could be an equipment issue and on a cheaper bike, that’s more likely. I’m just having a real hard time understanding how, what you’re describing could happen otherwise. Out of curiosity how much slack are we talking here?

My advice at this point would be to splurge on a new set of proper track nuts. Gut instinct tells me that may be where your issue is. I’d prefer to be wrong, but I don’t know how trusting I’d be of the quality of something like the axle nuts on a $300 bike.