Success! I've installed a new battery in my 6 running . Old battery capacity reported around 0.5Ah, new battery out of the box reports 2.96Ah and was around 50% charged, so letting it charge up now. Used a hair dryer to weaken the adhesive and @iFixit@mastodon.social suction cup and picks to get the back off. After removing the back, swapping the battery is quite simple.