I don't think you should increase your nicotine dependence to get TH. You could use 18 or 24 mg and yes 24mg will give more TH. But 24 is typically a level more used by ex-smokers of full strength cigs, and if it's too much nic for the amount of drags you are taking, you'll get symptoms like headache, rapid or irregular heartbeat, etc.
More nic will not give you more vapor. Vapor is dependent on the carto or atty and the amount of VG in the juice. More VG = more vapor and less TH
My Vapor4Life w/ Vapor4Life cartos give more TH and less vapor than my 510 w/ a Boge carto. Vapor4Life cartos are stuffed sort of tight compared to Boge blanks. But the Vapor4Life is slightly higher voltage than 510 and give hotter hits (heat also adds TH), especially with the WOW cartos which are lower resistance.
If you really want a lot of TH, buy a fat battery model (like Ego, Riva, etc) and use low resistance attys and cartos on it. The fat battery models have tough enough electronics to handle the increased current flow.