Hi Aerome. Your guitar has been under the bed for 18 years ( I feel sorry for both of you)!! Sorry not that you have a beautiful guitar but because your not giving it the attention you both deserve, enough said!
As you guitar has not been played much over almost 2 decades.....it's probably much better if you change the strings prior to doing a set up. It's important to only ever do a set up which includes a truss rod adjustment with new strings. I'm not sure if you did this. The correct gauge strings are important as well. Do you have a RIC nut-driver to turn the nuts ?????
Another thing that you should wish to attend to is to sparingly lubricate the rod thread and nuts. First De-tune the guitar, remove the TRC , turn back the nut on both rods until you expose the section of thread that that nuts had previously located and sparingly lubricate this area. Turn the nut by hand back and forth to work in. Even put one drop of lube on a clean rag and wipe the plate that the nut butts up against. Re tighten the nut by hand, and a little more with the driver, next re tune and do the final adjustment or adjustments. I would recommend this as you guitar has not been played and serviced for a very long time.
1 single drop of lube per nut is all that is needed, don't flood the truss rod cavity.
Yes you do have and underbow and your relief measurements indicate this.
There are different positions that guitar techs use to take these measurements depending on the guitar. It's maybe best to take relief measurement as such, put a capo on the first fret, depress 16th fret and put the feeler gauge under the 7th fret and take the measurement from there.
If you can change the strings to new ones, the lubrication and adjustment on the rods is not hard I'm sure you will get the neck with less relief. There is no such thing a perfect relief measurement for all guitars, but Low E .006 and high E .004 is in the ball park.
You have been wise to be a bit wary, yes your guitar has been in hibernation for too long. however I think you will get a straighter neck. New strings and a 5 minute service of the truss rods will be much better. You could also provide the string height measurement at the 12th fret ( not using a capo) the distance between the fret and under the string, take a measurement under the Bass and treble E strings.