I have a very ergonomic good brand chair (can’t recall the model but it’s a Steelcase) that I paid 200 for.
My GF is at home more than I am and stole it from me almost immediately. A month later I found another one on fb marketplace for $100! So now we both have one and her back pain has decreased tremendously.
The way I found mine was googling for used office furniture in my area. I found a warehouse that did that kind of business. They are tailored for small companies or startups that’s just want to put some tables somewhere and get started, but I made a cash offer and got the chair for much less than they offered to companies. It was a brown boring dead looking building with blinking fluorescent lighting that I would’ve never thought of.
I don’t live in a city with big fancy companies (some financial and medical but no tech or progressive offices) and they still had a good selection.
I hate the aeron everyone recommends. My legs go numb on it. The leap is ok but It was out of my budget. If I had blindly purchased one of those I would’ve hated my decision. What I’m saying is that buying after you sit on it is much better than buying one blindly.
For example, the standard height of almost every desk you'll get in the US is 29.5". This is just about perfect for someone who's 6 ft tall. If, like me, you are shorter than that, a proportionally shorter desk will change the way you use your chair.
Similarly, when I started using a split keyboard, this obviously increased the comfort in my shoulders and upper back, but everything is connected and I feel like it was a good move for even my lower back.
As far as chairs specifically, I picked up a Herman Miller Mira from a used furniture warehouse. It was still pricey, but for me it was worth it. I was able to get the lumbar support just right, and I'll sometimes adjust it through multiple positions throughout the day, because it is easy to do without even getting out of the chair.
I would definitely figure out the proper height for your desk. I needed to go down to 26" in order to have my arms resting comfortably without having my chair pushed too high.
Although I learnt this, it is hard to be always doing that, because bad posture (just causally leaning) feels better at the moment, but in the long run a good posture make you less pain.
Also consider invest in split keyboard and vertical mouse. I have carpal tunnel syndrome and it is a must for me. But the fact that such a set up makes people like me painless means that it is the most neutral position for your hand.
In general, avoid any tensions. In the neck, or foot, etc. when you are deeply focusing on a problem, try from time to time focus on your body instead and feel if there’s any tension. They are the things that’s killing you in the long run.
(Disclaimer: Not a doctor, just a fellow painful person most part of my body.)
That said, I have disposable income and like comfortable stuff so I have a Steelcase Leap v2, Idasen motorized Ikea desk, and I highly, highly recommend a Kinesis Advantage 2 keyboard. Next version of that keyboard, Advantage 360, is supposed to be released later this year.
I also bought a split keyboard: Dygma Raise. This has been by far the best ergonomic choice I made in a long time. Seriously, try a split keyboard. And I highly recommend the Dygma Raise, but there are other similar alternatives. The fact that it has less keys (and thus relies on layers, but also supports a trillion custom macros) is amazing (other keyboards work with layers too).
Not necessarily a be all and end all, but I find them pretty comfortable