I have done similar in the past successfully with my S4 using an OTG adapter cable (~$1 on Amazon). The cable allows the phone to serve as a USB host. I was able to use the My Files app on the phone to see the Garmin as a drive and see the files on it. I then used the manual activity upload on the RWGPS or GC websites viewed on phone's Chrome browser to upload the files to those sites. Although I have a Strava account, I haven't tried uploading to Strava via the phone.

Since then, RWGPS changed their website and it won't display properly on any of the browsers I tried on phone. Garmin is in the process of changing GC to the "Modern" version so that may present some issues.

That was with a version or two back from the most current Android version so I can't say for sure it still works.

As a bonus, I was also able to connect a mouse and keyboard to the phone individually or simultaneously via a 4 port USB hub and the OTG cable. Surprisingly, they worked pretty much as you'd expect.