Agree, lid the likely culprit.
Check the top of barrel that the oring sits on is nice and smooth, no nicks, cracks in the neck of the barrel or lumps on it. If lumps sticking up it needs paring smooth with stanley knife ( not likely that you'll find this problem though ).
Take the O ring out give it a good clean with alcohol and then warm water and check that has no nicks etc in it. If damaged replace it.
Use keg lube on nitrile or butadiene rings ( its silicone grease) and never use silicone grease on silicone orings it makes them deteriorate , which is a bit of a paradox I know. Use Vaseline on Silicone O rings.
I'm not sure what your oring is made of but pretty sure my kingkeg has nitrile or another non silicone o ring ( it's black ) .
I smear the lubricant on all of the oring and then put it back in the groove ( after checking that the groove / seat for it isn't damaged ).
Fill keg to near full and then put some gas in if you hear leaking / see bubbling tighten a bit more. If the lid is bulging up a lot that means the pressure is quite high and would be too high for serving, but get a gauge for that assessment.
The ptfe tape will only make the lid do up a bit easier as it's nice and smooth, the lid and oring make the air tight seal and the threads just hold it in position.