I am surprised that nobody has mentioned yet the (seemingly) functional Linux mobile OS/device: the FLX1s from furilabs (https://furilabs.com/).

User reviews seem very compelling, does anybody here have experience with it (especially if you can compare with unofficial Android ROMs, Sailfish OS, or Postmarket OS)?

Please note that user reviews are only out for the FLX1, not the FLX1s, which is about to start shipping soon. That said, as they went with the same SoC, hardware enablement is hopefully going to be fine and the software will work as well as on the FLX1.

I have their FLX1 device here, but I have yet to spend more time with it (instead of my postmarketOS daily driver I am writing this on). It's humongous (the s is going to be smaller), which is holding me back - but feel free to ask questions.

What's nice is that it's 5G hardware, and as they use the (4.19 IIRC) Android kernel they have a lot of hardware working which would be a huge struggle on mainline (think photography, finger print, ...). The Android integration via a Waydroid fork is also decent (but 'basic waydroid' is IMHO fine too, the hardware just needs enough RAM), and overall, it feels quite polished and the team is responsive in adressing customer feedback where they can.