[tgsovlerkhgsel]

A chair that works for you. Part of what makes this hard is that no chair works for everyone, and very small changes can completely change the outcome.

Try to find a shop that lets you borrow a chair for a few days before you buy, or has a generous return policy - there really is no way of knowing without trying, and trying for several days.

Or buy a bunch of cheap chairs and try those.

Also, definitely adjust your WFH setup to be ergonomic - correct desk height (manually adjustable desks aren't expensive at all), correct monitor height (e.g. by putting a stack of books under the monitor), and of course a proper keyboard. Also, try to move and don't stay in the same position for too long.