Is it possible to completely wipe smart devices/wearables and run 100% open-source, offline OS?

With recent concerns over smart devices, wearables (screenless fitness bands specifically), and Android phones constantly collecting data and listening, I’m looking for alternative options.​Is there a way to buy the hardware, completely strip the stock OS/firmware, and run open-source, locally-stored software instead? I'd like to use a screenless fitness tracker where all data stays strictly on my device. Has anyone successfully flashed open-source firmware on popular wearables, or are there specific hardware brands built for this?