Using Bluetooth LE ads should in theory work on any device which can send those ads. However I only have example code for Linux (tested in Debian 11 and Raspberry Pi OS) as I don't know how to use Bluetooth LE on other devices.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Mark_M
Like some other people here, I'm interested in whatever will work with an iPhone. (Android and Gadgetbridge are not possibilities.)
Which devices will provide the synchronization?