Well lets start with the metallic flavor that you are experiencing this is a sign that there is oxidation going on
Firstly I would assume that you are using Cornelius kegs (post mix) make sure that you disassemble all the parts clean the popper valves replace all washers if they appear faulty clean your 6 mm stainless tube in and out (siphon) use normal pool chorine as a sanitizer but not neat and always be careful when handling it as it eats the dye out of your clothes and wear eye protection at all times
Flush all parts completely with tap water reassemble pressure test for leaks before filling with beer this is a givme
When you have established that all is pressure tight use your beer connection the black one to connect to the beer out side of the keg and your co2 bottle then slowly purge through the spear and vent through either the co2 nipple or the relief valve we do this as co2 is heavier than air and will push out any oxygen that is in the keg
What I do next is I use my pump to transfer beer from the fermenter to the keg if you don’t have a pump place the fermenter higher than the keg a siphon it through a co2 flushed tube make sure that the end of the tube in the keg is on the bottom so that it doesn’t agitate the brew
Once full cap and force carbonate to desired co2 volume make sure that your beer is as close to 0 degrees as possible as the co2 enters the beer so much quicker also what I do is set the regulator up to say 400 kpa and force co2 down the spear side and shake the bugger for some time you will know when enough has entered the brew as the bubbles will slow in sound if you happen to over carbonate supply blow some of the top via relief valve
I personally find that my beer is better from the keg than the bottle but that is only preference also I hate washing out bottles, glasses are so much easier
Cheers to more beers speedie