I should know this, but my mind went completely blank and even more confused looking over FinCEN Rule 2/2020. How should a CTR be completed for ABC LLC that is a single member LLC using the SSN of the owner (the owner is also the conductor)?

Two part I's. This is not a sole-proprietorship. It is an LLC.

Person conducting the transaction is item 2b
Person on whose behalf the transaction is conducted (the LLC) is item 2c

You will list the owner's social security number for both Part I's.