What would be some good promotion board study tips (and on what topics should I focus)?

Your assigned and qual'ed weapons
PT
670-1 - AR AND DA PAM
Unit history
Creeds
CoC and NCO Channel
CBRN
your short bio/summary

Those will get you started.

Honestly, just search threads on here. This question is asked weekly and there's a ton of good answers from the last few years.

Know your unit's history as well as current events. Know your job. As far as study material, there are study guides for the board (even the Ranger Handbook is a good source). I trust you have a mentor; tap their knowledgebase.

Now, for the stuff not found in books...

It may seem silly, but practice your confidence. It matters! Watch yourself straight in a mirror, and have a buddy (or yourself) record you in profile doing a practice run. Are you standing up straight? Are you making direct eye contact with the imaginary board in front of you? Do you sound like you speak with clarity and authority? If you answer no to any of these, work on those presentation abilities, because the board not only want to know you know something about being a soldier, but they want to know you'll be decisive in your decision making.

Good luck.