Riding into pain will not work. Don’t do it. First do no harm.
From the photo your self-description of being in decent shape seems reasonable. Which means it should be possible to get out of this hole. If you can ride three hours indoors you are not even close to a hopeless case. Staring down at a handlebar mounted digital display for three hours is not going to help that kyphosis. You could start by riding indoors looking up.
Except for the kyphosis riding position looks OK. Try (I mean try, it is a possibility , not a prescription) putting the saddle down a little. Less leg extension might give a little free play to lower back. And ride above the brake hoods or even on the tops. If riding with hands above the hoods is immediately and obviously successful/less painful then go ahead and get a shorter stem.
Drop bars mean hard work for the neck. Cyclists plain have big thick necks. If your neck doesn’t cooperate you are not riding. Or you ride with flat bars.
Problem with online workouts is you need a good hands-on evaluation first.
Chiropracty is an obvious route for a stiff back. It works when it works. In an area where yoga classes are scarce you probably have fewer choices. Basic recommendation for chiropractors, or any other body work, is you feel better first time or you move on. Rolfing would be good for your problems if any rolfers are nearby. Again, you either feel better after first session or keep your money in your pocket.
If there is a sports orthopedist in your area make an appointment. Sports orthopedics is a board certified specialty. They do no surgery and work with physical therapists. Doing no surgery means they volunteered to make far less money than surgeons. A good beginning basis for trust. If there is a sports orthopedist available this is one they will fix.