Is it worth doing a battery replacement this late in?

If it's a matter of opinion, I personally wouldn't waste money that could go to something else on that replacement. Unless you can find a way to get more value out of the phone selling it with a replacement than not -assuming you intend to sell it- or if you want to give the phone to someone in the short term, I'd just wait and make the change. If the cost is low and you find that you have no alternatives for the interim, weigh the matter accordingly.

If you're sure it's just a battery issue and the device is in good condition, it's pretty surely still a good phone with decent resale value, so keep that in mind.