First thing I'd do in your situation would be to re-think the August hill climb. It'd be tough to recycle in 4-5 weeks for anybody, much less when you're over 70.
Since you seem to like climbing, perhaps the Cheaha Challenge or Mt. Mitchell in May, or Cherohala in June (assuming, of course, they run on the same schedule as usual in 2021) would give you plenty of time to recover from the first event, and then train and taper for Six Gap. Of course, you'd probably want to start training hard earlier in the year.