A few weeks ago I got the firmware to a point of stability which now I feel more comfortable sharing. You can now find the firmware here:
The firmware is actually a simplified version of bicycleCompanion.
Be sure to follow the instructions on how to upgrade, to reduce the chance of bricking (there's still always the chance, so be prepared for that). Anyway, I've been using this firmware for a couple of weeks now and found no further issues. It is still a basic watch firmware with no fancy features but I focused on stability and battery life. As a result I get about 9 days of casual use (check the time a couple of times per day by turning the backlight on).
There's still some quirks like charge completion detection (doesn't work, so you'll have to see when the charge voltage appears to remain constant for some time). Also, the battery voltage is kind of a guess since I don't now the exact resistor divider ratio used to measure it.
I think that with current state of firmware you could explore adding more features/screens. You could for example write a driver for accelerometer and do some tap detection, use the internal SPI flash or even the heart-rate sensor.
At the moment I don't think I will do more with the watch as I'm actually awaiting for a SMA-Q3 smartwatch, which has a lot of very interesting sensors and better input support. I will most likely focus on that. Also, I hope to finish BLE support for nRF52 on NuttX eventually which would really open possibilities for these watches (and other ones, such as PineTime).