I think the problem might be more fundamental and not significantly related to the styling of the button.
The vast majority visitors are just consumers and leave when they found the answer/guide they were looking for. Also, creating an account to ask a question does not lead to retention of valuable users.
That being said, you might want an internal metric to measure what percentage of logged-in-users reach the bottom of the page without using the “completed” button.
Unless you’re already repair (and open-source) minded, there aren’t a lot of incentives to become a meaningful member of a repair community.
One thing I can think of is to put more emphasis on the devices you own (my workbench). Unlike Wikipedia for instance, we can make a relation to the physical world of someone.
- the workbench could be more accessible, part of your profile page for instance (not public obviously) or a dedicated page, and not just in a dropdown
- easy access to changes for your devices, “a device you own has a new guide” or “a device you own has received X guide updates”, either in the dedicated page combined with notifications, or in the monthly mailing
This in itself could help with retention, not with crossing the initial threshold of becoming an actual user. I’m not a marketing expert, but IMO Ifixit is more marketed as a part in the repair movement, and not as a helpful tool for regular users with day-to-day problems.
Perhaps the devices-you-own overview could be expanded and marketed to actually help regular users, for instance to get reminded of yearly maintenance jobs or just to add a note of where you stored or lended it.