I'm curious about the level of expertise you need in order to do this. I also have a cheap bracelet and I wanted to do the same thing, but reading this article seems like you need to be an expert in electronics/lowlevelprogramming.
It’s something I find refreshing to read and like to state when I can: none of this was easy for me. But it was definitely fun. Having no deadlines helps. Having no practical goal and just enjoying the process was also refreshingly nice.
Recognising the I2C/TWI pins for example is a really quick win when looking at the OLED display. Otherwise, there's nothing much to do but spend a day googling - but you would find it with tenacity.
The difference in experience here is in turning it from a year long learning project for a beginner to "uncountable hours over several weeks" of exploration for someone a little more seasoned.
The soldering is quite fiddly, though!