I work from home and had back pain for maybe a year. It effectively went away after I switched from a crappy budget office chair to a quality ergonomic chair. I opted for the Steelcase Leap.

These chairs are over $1k new, but it seems you can find them used for around $400, maybe even less if you have a local used office furniture store. The downside is that you're giving up a good warranty by purchasing it used, so if this is a 10+ year purchase then it may actually worth be getting new.