So is there a consensus that the valve adjustment is what I really need at this point (on top of the plugs)??
When was the last time you had it done? Was it done less than 105K miles ago? If not, yes you really need to do it. As I already mentioned, it sounds like you've been skipping your regular maintenance, and your mileage may be one way you're paying the price. Also you should really do any of the rest of the scheduled maintenance services that are now overdue, including the transmission and other fluids, etc. Some could affect mileage. And even items that don't affect mileage - like changing the brake fluid - should not be allowed to extend beyond the recommended service intervals, for all kinds of reasons (safety, reliability, etc).
What you really need at this point is all three things I listed above: filling with premium fuel, bringing ALL the scheduled maintenance up to date if any of it is overdue, and cleaning out the EGR valves and ports.
If you're going to keep using regular fuel, then you may as well give up, since you're not even doing the most basic thing you can do to get better mileage and take proper care of your car.
should I do this at the dealer?
That's up to you. Any good independent mechanic - and I stress the word "good" - can do all of this work. You don't have to do it at the dealer. Independents are often (not always, but usually) less expensive than dealers. The quality of work varies at dealers, just as it does at independents, so what's key is finding a dealer or shop that knows what they're doing, is honest, etc.
Two things I haven't seen you mention yet: tire pressure and tire *weight* can be a big factor on mileage too.
Tire pressure can be a big factor on mileage because it affects rolling resistance. Tire weight, no; any difference will be insignificant. Maybe on a 2000-pound sports car where you're looking for hundredths of a second per lap, but not on a 4500-pound SUV driven on public roads.