The last thing I saw on this is that Garmin charges a one-time fee of $5,000 for access to their API…
However, I read somewhere else, that Garmin dropped the fee. This page very well may bare that out: https://developer.garmin.com/health-api/overview/ (details of what’s covered in the API are at that link too)
GET THE API
Access to our Health API is free to approved developers.
I guess the trick is becoming an “approved” developer. 
But the one thing I am most curious about is, would switching to MapMyFitness or Runkeeper help? I find CityStrides more enjoyable than Strava, though I would miss the metrics. But maybe they are there in the Garmin app? I was using the Strava app before I had my Garmin, so kept rolling (and running) with Strava. If one is “better” than the other, which one? I have no experience with either.
[edit] It was hjkiddk that originally posted about the API is free, in the “poll thread” It was bugging me that I could not remember where I saw that.