A am very happy to announce that the longest test run so far - three weeks in a hot office - shows a drift of around 45 seconds.

This is far exceeding my goal of "under one minute per week" :)

The calibration data is taken from a one-week GPS-backed trial, so I expect it to behave slightly better if I re-run the analyze script based on these three weeks to output newer model parameters. But, from now on, I consider these changes to be cosmetic only.

I had to re-start the runs two times because of the I2C communication to the temperature sensor. The 1MHz from the external GPS-backed reference oscillator induced some serious noise, so I decreased the I2C bus resistors slightly.
This was enough to make it mostly work, but for a time-piece that should run a long time, this was not enough. I had also to detect a stuck I2C bus and a consecutive I2C bus recovery (see also i2c-lock-up-prevention-and-recovery).
And even that was not enough; sometimes the BME280 seemed to stop the measurement-cycle, which can be inferred by the registers having a certain default value. After detecting that state, and re-starting the continuous measurement mode, it finally ran through, even with people moving about in an office with carpet floor.

To finalize the project, I want to crimp a custom cable to contain power, serial connection, and the (now optional!) external reference oscillator in one connection. Additionally, I'd like to make the model parameters run-time configurable (in the NOR flash) as a finishing touch, and perhaps I will improve the info of the watch-faces. Let's see.

But first, after many requests, a video where you can actually hear the tuning fork when I take off the glass hood (and stop the fork by touching it):

Oh, and by the way: Courious Marc just recently released a video about the HDSP-211X Display that I use. I agree both on the beautifulness as well as the fact that it is hard to film: www.youtube.com/watch?v=JXACON1XkPI