I've replaced the battery on my 2015 MacBook Pro with one from iFixit [1]. They're a huge pain though, since they're glued to the inside of the case and requires using a solvent to get the battery out.

It has held a charge for the last few years well. The other issue I had with a laptop this old was cooling. Ended up swapping out the fans and heatsink/heatpipe and am expecting to get another couple years out of it.

1. https://www.ifixit.com/products/macbook-pro-15-retina-late-2...