Late 2023 I went through all of GameDev.tv's Programming Design Patterns For Unity. It was very helpful, teaching me new design patterns and helping me understand ones I was already using.
My biggest takeaways were the ten "coding commandments":
Thy code shall read like English (even without comments).
Thou shalt take time to name things well.
Thy classes and functions shall have only one clearly-defined job.
Thou shalt use as little state as possible.
Thou shalt not refactor before the 3rd repetition.
Thou shalt not use static and global (without very good reasons).
Thy classes shall be highly cohesive.
Thy classes shall be loosely coupled.
Thou shalt use composition over inheritance.
Thou shalt follow the Law of Demeter.
And my favorite quote:
There are 2 hard problems in computer science: cache invalidation, naming things, and off-by-1 errors.
 - Leon Bambrick