What about #100daysofcode challenges? Good or nah?

Nah. Choose your own adventure is more fulfilling. I have been known to pick and choose entries from advent of code though, they tend to be more interesting.

When you have around 5-6000 days of code, you tend to not worry about these things.

Depends, are you a student? Events, challenges, competitions, ... are a great way to challenge yourself to learn new things. Once you're done with that, your job should provide enough of a challenge to keep you moving forward. After 8 hours at work, go find something else to do, don't let your life be all about coding.