Many of the best juices out there use Capella and FlavorArt flavourings. There are a few other flavouring companies out there too but to me Capella and FA are two of the big boys.
As you said, narrowing down your ingredients to find out what is making it taste rank is the right way to go. With all those mixes tasting bad then it does sound like it might be the VG or PG but they are pretty inert substances. Are they clear? Do they smell at all? If not then I would be surprised if they were tainting your juice flavour at all. As you said before, do a mix of 50/50 PG and VG only and see how that tastes. The add some nic to it to make is 6MG and see how that vapes.
You are mixing at 10% flavouring, which is relatively low so can't see that it is these (especially as using a range) that are making the juice harsh - sounds more likely to be the nic so have a test and see.
I must say you are a rare one vaping at 28mg, that is very full on nic, even in a Nautilus! Just to double check, are you aware of the nic limit of 18mg coming in from May? Time to work to reduce your nic level or invest in a freezer and a shit load of nic if you can't face dropping down, let us know if you are unaware and will post more info.
If you find it is the nic that is making the juice taste bad then let us know and we tend to recommend Dark Star.