The thing that outperformed every purchase for me: changing position regularly beats finding the correct one.
Eight hours in a good position still hurts. The load is the duration, not the angle, and this is why people set up a perfect workstation and still ache - they have optimised the posture they now hold rigidly all day.
What that looks like in your constraints, where you cannot adjust anything:
- Stand up every half hour for one minute. A timer, because you will not remember. This one habit did more for my upper back than any equipment.
- Take calls standing or walking. Free, and it breaks up the longest static blocks of the day.
- Deliberately change your setup through the day: an hour at the counter with the laptop on a stack of books is a genuinely different load from the dining chair, and the variety is the point.
Also: an ache that arrives at the same time every afternoon is usually accumulation rather than position, and an ache that is only on one side usually is position. Yours arriving mid-afternoon fits the first pattern, which is why I would try this before spending anything.
If something is painful rather than achy, or it is not settling overnight, that is worth taking to someone who can actually look at you rather than to a thread.