Definitely! $500 is a very good price for a (new) chair of that quality.
As for the mounts, for most people the main advantage is opening desk space underneath your monitors, but for some people there is also a definite ergonomic advantage. I’m a little bit taller than the US average, and I find that a lot of stock monitor stands don’t adjust high enough for me when everything else is in an appropriate position. Even a cheapo Inland arm gives the extra inch or two of height that I need. For arms, the low-end ones do tend to slip down a bit even when putting really tight tension on them. The more expensive (~$100) ones don’t do that.
Although if you’re ok with the ergonomics of your current monitor configuration, I’d instead echo epigraph’s suggestion to look into a high-end keyboard first if you don’t already have one. I went mechanical like two years ago, and I wish I’d done so sooner.