Replaced with battery from iFixit. Remains to see if capacity improved. Volume down button did not work after the replacement. Upon further inspection, I found a dislocated resistor on my desk while cleaning up. Sad days.

Opened the sim tray with a 0.1” pin header. Opened back cover clips with the back lever of a pair of nail clippers. Removed battery by heating it with a soldering station heat gun on the lowest setting for a few seconds, then prying it up starting under the flex cable. Experienced the “terrifying crack” mentioned by another commentor.

On the volume button, looks like this reddit commentor had the same issue: https://old.reddit.com/r/nexus5x/comment...

So please be careful with the volume buttons if you’re doing this replacement.