[brudgers]

[random advice from the internet]

The best way for a beginner to learn X is to not spend time on finding the best way to learn X...

...and instead get to the hard work of learning X.

build my own Smart watch based on a Cortex M33 or M55. QSPI flash, I2C sensors, bluetooth, power management, some DSP

Just start now because actualizing that word salad is a muliti-year project. Alternatively, buy a smart watch instead and be done with it. "Some DSP" is at least a four year degree equivalent.

used a HAL library and it feels like cheating

Cheating how?

Good luck.