So true. You can sit on anything with any awkward position you want, sleep 3 hours a day, and don't feel a thing.
Then you turn 30...
As for ergonomic gadgets those never helped, simply trying to keep myself strain free - like instead of having split keyboards holding my hands in angle over the keyboard without bending my wrist -, relaxed, but trying also holding myself not flowing down of furniture. In some kind of balance.
It is funny though that Aeron is brought up as an example of a "universal" design: they are admittedly not universal since they offer 3 sizes (A, B and C). When looking at chairs, I've only went with their Embody because of the fluid back mobility it allows when sitting, though I still wonder if I should have gotten Aeron C (I always found it sad they did not continue with the practice of differently sized chairs with Embody which I love). I've also have a sit/stand desk that I switch between every day for the last ~10 years, but I've still developed back problems.
Why do I say all that — at 6'5" (195cm) I believe to have pretty strong core and legs, but I developed back problems when I got a kid (herniated discs, with a nerve getting pinched in the lumbar area recently). I live in a world where everything is too low for me, and I have to take an unergonomic position to achieve many things. Especially with kids (baby cots, strollers, they are really short, but everything, really).
A dining table chair has a seat length that generally allows me to only rest most of my bottom on it, but basically nothing of the thighs (which are also longer than average even for people my own height), which means that majority of my 90-100kg weight is concentrated on a small area. That hurts, so I have to move around and change the area under pressure.
I had to stop playing jumping sports (I jump around 30 inches/75cm high when not in regular exercise, so that's how far down I "fall" when going down) because impact on landing was too strong and it started hurting in my back. Yet after being in bed rest for 5 days for a pinched nerve, I went for physical therapy and could immediately hold plank position for >1 minute. "Your core is strong, so it is probably genetics", my physiotherapist would say. It's not only genetics, it is the fact that I am outside the design range for all the products that people use every day (I am probably among the 3% of largest humans and even the widest covering "universal" products cover from 5-95%: I can imagine very short people to have similar issues). I can barely find a car I can comfortably drive (if I push the seat back enough so my thighs rest on the seat instead of being in a squat, steering column will frequently not come out long enough, and my shoulders will hurt). I might fit in a Mercedes S-Class, but sorry, I can't afford it (though even that is designed for comfortable sitting in the back).
And I thought that's what the OP is about. Are you averagely sized, or do you maybe have some parts extra long or extra short? Or are your shoulders narrow or maybe extra wide (eg. I can imagine someone with very wide shoulders having more trouble with laptop keyboards)? Even the way our elbows and tendons connect is very different, so relative position of every limb could be taken into account to develop non-universal products.
I guess you have to find where you are "unusual" in your body constitution to really emphatise with the OP and how "universal" design is not the way to go when designing products for the physical world.
The same holds for the virtual world, but for some marginalized groups (at least in the past, like people with disabilities), legislation is required to make sure the basics are available to them. But generally, the market is ripe for less universal products.
That is, I can't lean on youth. But I also don't have near the pains mentioned on so many message boards.
Now, do I have trouble staying focused? Oh yeah. Staying motivated? You bet. Haven't exactly been a pleasant year. But my seating seems to be the least impactful thing going on right now. :(
You are doing well by standing. Keeping your core strong will help.
The thing you have to ask yourself is - are you using willpower to overcome bad seating? If you have good ergonomics you will maintain good posture even when you're tired. The idea is to balance your body so it stays in place. Keep your head balanced, your arms relaxed without hunching your shoulders, your wrists at a good angle. If you're looking to the side or up or down, you're going to use muscles instead of balance to maintain your position. (Laptops are not ergonomic and are responsible for long term damage to your body - there are no osha approved laptops)
If you have your wrists at a bad angle, on the hard desk or across the sharp edge of a keyboard, the little ropes and pulleys that get your forearms to move your fingers will wear out. (I use a wrist rest to maintain position, and keep everything warm and comfy)