To the OP: don't overthink it too much. Just go out with an instructor and see how you feel about it. For what it is worth, my flying history somewhat parallels what you and others that have posted. Maybe you will glean some useful information from my story. Or maybe not.
My flight training started in 1965 in Millington, TN while I was a lowly Navy E-3. In the next 18 years, in spite of long overseas hiatuses, I acquired about 1800 hours of PIC time, along with a commercial certificate and instrument rating. My flying stopped abruptly in 1983. As of this year, I am flying again, after a 34-year layoff. The process started when I took the AOPA "Rusty Pilots" seminar (it counts as the ground school portion of your FR). In March of this year, I scheduled a demo flight with an instructor; my goal was to see if flying was too "alien" me after all of the time that had passed. The instructor threw out the surprising idea that if things appeared to be going well, we could turn it into an instructional flight with the possibility of my being signed off on a FR. We flew out to the practice area, did some turns (medium), turning descents and climbs, and did several dreaded stalls. Coming back into the pattern, we did a few touch-and-go's, a full-stop, then another full-stop. 1.1 hours logged, FR signed off! Legal? Yes! Ready for prime time? Not quite.
As of September, I am the owner of an airplane, albeit a Light Sport. Light Sport will probably my continuing status. After 5 hours of dual to satisfy insurance (and myself), I flew the plane to Texas from Virginia.
So, what observations might I make about the process?
1) I was concerned about all of the changes that took place over the decades. Mostly, I have found the changes consisted of terminology. The old "TCA's" and "Airport Traffic Area's" have been replaced by the Class B, C, D airspace. When you distill it down, not much else has fundamentally changed. If you will be flying into controlled fields, the airport markings seem to be much more codified (but still with a lot of variations from field to field)The moral: get yourself well-schooled in the changes. The AOPA website has great info. Also, there are several youtube videos that do a show/tell about today's airspace.
2) I was concerned about the technology changes i.e. "glass panels". I am a technology guy, but a couple of demo flights in an all-glass paneled airplane was was daunting. I did not like it. I felt that trying to learn totally alien instrumentation at the same time that I was trying to re-capture flying expertise might be counter-productive. I felt that it was more crucial to re-gain basic flying skills than to have to figure out how to change the altimeter setting while in the air. QUESTION: How many button-pushes does it take to change the barometric setting with a Dynon panel? ANSWER: 9. I solved my dilemma by buying a plane with the familiar analog gauges (it even has a VOR w/ localizer!), and I bought a tablet computer with GPS and loaded up IFly onto it. In 2-3 days of practice with it, I felt I could handle it adequately. I used it on the flight back to Texas. But I always had a VOR frequency tuned in- just in case.In this way, I had the type of instrumentation with which I was familiar, and I had the new-fangled GPS nav on the seat beside me. I will say that having a ton of information such as airport data, nav data, and comm frequencies, and so much more made things so much easier. The moral: re-establish your flying skills, then decide how much technology to employ (and in what form) .
3) I was surprised at how quickly long-dormant "muscle memory" returned. At one point in the A/C checkout, the instructor noted as to how I had automatically adjusted for the crosswind by going into a forward slip. I never told him that I had not even thought about it, nor realized that I was doing it. After several (bounce-in) touch-and-go's, he also noted that he could see my old training start to kick in. The other side of muscle-memory: in the AA5 that I was fortunate to own, the flap switch was located on the console between the seats. On this plane, it is on the panel. Still, after about 25 hours in it, my hand sometimes goes to the console first. Moral: skills will return, but you will have to actively correlate what you (think you) used to know with what you are experiencing/learning now.
4) My biggest obstacle to date has been cockpit management. I am still conflicted by how I remember having done things versus what I find myself currently doing. I think that I used to rely on memory for various aspects of aviating, for instance stall speed at various bank angles; now I feel more comfortable with cue cards stuck to the panel as reminders. In planning the flight from Virginia, I did the old pencil and course line approach on the sectionals, but also using IFly as backup. But I had forgotten how tough it can be to re-fold a sectional while in bumpy air. That gps nav on the tablet mentioned above sure came in handy. I am still working on routinizing the common procedures, such as pre-flight, start-up checklists, shutdown, etc. I am also working on re-developing the "little" habits of flying, many of which I do not know I am missing- until I discover that I am missing them (the old habits).
So, if you once had a solid compendium of flying knowledge, skills, habits, and capabilities, with time and focus most of those those abilities will probably return very quickly. Keep in mind the pace of your expectations and new learning. Initially, we all learned a few steps at a time, with slowly advancing knowledge and capabilities. Every flight (whether in training or as PIC) added incrementally to the body of expertise. Now, the tendency may be to try to re-acquire that same level of expertise without sufficient current experience. To a degree, we are re-learning how to be a competent aviator; but we still have a learning curve to go through.
As others have indicated, be careful about your medical. Don't assume you are good to go just because you feel OK. There are all kinds of gotcha's that could cause you to not be approved for a 3rd class. If you become permanently not-approved, then your flying days are finished. Even Light Sport is not an option then.