Love your questions and please keep them coming. They are useful for other community members as future reference and trust you'd be able to master QuickBooks Online in no time and might start answering questions here.
For this one, you are right! Purchase order and name of sales reps are the best to use for custom fields.
I understand the mapping process may seem less than ideal at the start. We are constantly in look out for feedback on how we can improve this experience for our customers, so please give us some feedback on your experience as well.