First I'd properly identify the source of the drag. The list of solvents that you've used already contains 4 very effective cleaning and degreasing solvents. So it's likely that you are not working on the correct spot. It could be at the shift lever or in the cable and housing or in the derrailleur. Since you've cleaned the lever overly well already I would suggest you concentrate your focus on the cable and housing and on the actual derrailleur. What you want to do is isolate the cause.

Disconnect the cable from the derrailleur. Push the derrailleur towards the center and let go. It should snap sharply and cleanly back to the outside position. If it's stiff you have your problem. If it's stiff and you've already cleaned the snot out of it with those other things then it's likely that your derrailleur is bent and it's the friction from this damage that is causing the issue.

Moving on to the shift lever. Pull the housing out a little so you can get a grab on the cable. This will isolate the housing and cable from the shifter. While holding some tension on the cable run the shift lever up and down noting the feel. Each click stop going both ways should feel crisp with only a couple of pound of cable tension needed to provide correct cycling. If it feels gummy then there's your trouble spot. Squirt in the brake cleaner and WORK THE CONTROLS to help it clean out the crud better. Nothing will do the job on it's own, you need to work the pivots to encourage the solvent to get into the crud and flush it away. This last bit applies to both the shifter and derrailleur.

If the two ends are good then that leaves just the cable and housing. Over time the cable cuts into the plastic inner liner and can get quite stiff. If you have not changed the cable and housing in a long time then it's very likely that this is the source of your trouble. Buy some new housing and a new slick finish cable and enjoy new found shifting performance.