Viewing its condition I am
wondering if the best course is to have a roofing contractor sweep the
gravel clear and apply an additional coat or two of hot roof tar or some
other product and then spread gravel again? Maybe that will make it last
another 15 years by which time I'll be heading towards 90!

Nope. You'll get another few years, but not much more. It's past its
useful life.

Honestly, I don't think I'm deceiving myself when I say the roof looks that
good. Point is it IS 33 years old. I'm prepared financially to do something.

Then do something. Get quotes and advice from at least three roofers
in your area. If they say recoat, fine. If they say reroof, also
fine.

It isn't in bad shape.

Do you want to fix the results of a leak when you do figure it gets
bad enough to require redoing?

It's your last roof, ever. Take care of it now and live worry free.

Jeff