Sorry for the thread necromancy, but I had an update and wanted to share my progress with fellow Linux fans.
TLDR: No official Linux support for any wearable fitness trackers for those of us who run (and run with) Linux, but Garmin devices in general are a good bet, since the software can be run using wine. And even if you operate without a client, the devices' file systems are mountable, so you can manually copy the fitness data and retain it for your own analysis without handing your data over to Garmin or any other third party.
Not too long ago, I saw that Chris Titus wrote a tutorial "Garmin Express on Linux: Step by Step Guide," where he explains how to create and configure a custom wineprefix for the Garmin client. It looked promising, but after getting Garmin Express up and running on my Pop!_OS desktop, I still wasn't sure if the client would detect the device, which I was prompted to plug in. No video tutorials I've been able to find show the device actually syncing or connecting in Linux, and buying a Garmin watch just to see if it would be detected seemed a risky purchase.
I decided to try talking to my local running shop to see if any of them had experience with my dilemma or knew any local runners who had. They got in touch with a local Ultra (races >26.2 mi) runner, whose similar use case stood out, as he's an avid Linux advocate who has to deal with syncing his data, as well as coming up with solutions to recharge his devices while in-use, as his run/hikes often exceed 24 hours. He gave them an overview of his process and preferred gear, so I was able to purchase with greater confidence.
So I gave it a shot with the Garmin Instinct, and the device syncs using Garmin Express on wine successfully (See Titus' guide guide for configuring the wineprefix). I can also access the files directly from the watch's mounted volume for analysis and tracking with pyTrainer, which is handy if you don't want to share your training data with the likes of Garmin or Strava (though I personally derive some training benefit from the social aspect of the latter). So I'm declaring a qualified victory, as I can now access all of my training data using only Linux, without a Windows or Android device to bridge the gap.