In 2006 I purchased a new Nikon D200 or dx camera and eventually purchased 3 or 4 lenses for it. In 2014 I bought a used Nikon D800 and slowly purchased FX lenses for it. However, some great shots can still be made with your DX lenses on the D800 and in full frame mode as well. Using the Tokina 11-16mm lens in FX mode can be done WITHOUT vignetting if shooting at 16mm focal length. With this combination I was able to create this 5 photo HDR shot of Upper Antelope Canyon that is very sharp and detailed which btw has also been chosen for the inside cover of the Corel Paintshop Pro 2019 software box! I still have and use the D200 (mostly as a back up camera). My only regret moving from DX to FX is that I did not do it sooner! It's as if I went from driving a VW bug to driving a Cadillac or Lexus. I am 64 years young.

I'm a little younger than you, 66, but after almost passing away (8 hours before they pulled the plug - pneumonia after a bypass - miraculous recovery), go with the FF. I have both, d7100 and d750, shoot events, second shoot for weddings, shoot headshots for local actors and I will take the FF any day of the week for what I shoot. If all I wanted to do was shoot in decent light, outdoors or with flash indoors, I might not. However, tomorrow may never come my brother! Go for it. Buy some used good glass - like a 70-200 or 24-70 2.8, maybe an 85 1.8 or whatever else you want and have a blast. Consider a used d810 for those 36mp for landscapes. I did a model/actor shoot a few days ago and will be posting pics with the d750, horrible light, no strobe/external lighting and take a looks at the pics.. I could not do this with a the crop sensor you have. I have a d7100 paired with the superb 17-55 2.8 which I have shot weddings with. I will not complain about the weight either.

You didn't mention what your favorite photography subjects are or why you are thinking about a FF body. Why eat the expense of a full frame if you don't need more pixels or better low light performance? Then there is the cost of new glass. If you do need a FF body, look at the D750 (24mp) or a D810 (36mp), both of which have been severely discounted lately, especially if you buy used. If you have money to burn the D850 (45mp) is arguably one of the best FX dslrs ever made. If you are going to stay with DX format look at the D7200 (24mp), D7500 (20.1mp), or D500 (20.1mp). Again, there are some great deals out there for used DX bodies, especially for the D7200 (around $500). I picked up a pristine used D500 with a power grip for a great price. The D500 is arguably one of the best DX dslrs ever made. For FF lenses, look at the trinity of the Nikon 12-24mm f2.8, 24-70mm f2.8, 70-200mm f2.8 and add a 1.4x teleconverter. That will cover most anything you want to shoot unless you are really into wildlife and birding, then you'll want to reach out to at least 500mm and the Nikon 200-500mm f5.6 is a good choice. Each of these lenses will set you back from $1200 to $3000, but can be had for 20% to 30% less if bought used.