Drashkum, your photo says a lot. First, your arms are proportional to your legs, longer than average. They push your back upright, higher than it wants to be. Second, you're holding your head down, almost lower than your shoulders, and your back is hunched. That's your back muscles trying to find power that's not there because your hands aren't in the right position for pulling. Third, you appear to be sitting in the back seat. That's because your back is upright, throwing your center of gravity back.

My advice is to accommodate your long arms by getting your hands lower. Use the original stem, flip it down, and get your brake hoods at least a few cm lower than the saddle. Second, look at your fore-and-aft weight distribution. Your center of gravity should be roughly over the balls of your feet. Lowering the torso will get you part way there, but you might want to move the saddle forward a bit too.

I have similar proportions, and I never got comfortable until I lowered the handlebar so the brake hoods were about 7-10 cm lower than the saddle. This has been my general position for about 45 years. And I only experience symptoms similar to yours when I demo bikes with the so-called "endurance" geometry. My hands, arms, and shoulders are pushing up while my lower back is pushing down.

Ultimately, you want to find a position that feels balanced, in control, nimble and athletic. This is about finding where your hips and hands want to be, not following prescriptions.

For models, look at photos of old lanky pro cyclists. Eddy Merckx, Fausto Coppi, Ercole Baldini, and Hugo Koblet were my models. Looking at riders from the 50s through the 70s works better because their positions were more moderate than contemporary pros'.