Have you tried using a brand new tank? I had similar chemically taste persisting in my tanks (Halo Triton) and I think the culprits were uncleaned coils and also dry hits due to pinched wicks. No matter what I did, I couldn't get the nasty stuff out. The tanks are going into the RIP bin.
Also, all of the heads you tried might have some nasty chemicals in them. Have you cleaned them?
An idea for you, in a new and cleaned tank with cleaned heads, try vaping x%VG+y%PG+nicotine and no other flavors. Vary x and y and see if any of them cause problems. Good luck.