I don’t know how experienced you are, so I don’t want to seem like I’m talking down to you but I can think of a few tips that might help:

finger tighten the nuts before you tension the chain, so you don’t have to turn the nuts as many times, when you start torquing them down.

tighten the non drive side nut first, but of course don’t tighten it all the way down.

try to tighten the nuts back and forth so you get even tourque on the axle from both sides

if you’re not using a chaintug or something else, try holding the wheel centered between the chain stays as you tighten the nuts.

hope those help

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.

If your track nuts do not have built-in floating washers, it is almost impossible to tighten them without the axle moving in the dropout when you tighten it. If this is the case, either buy better track nuts with built-in floating washers or install some thick flat washers between the nuts and the dropouts.