This is really a non-issue imo:
- The expansion doesn't end on a cliffhanger at all. Plus there's a reasonable amount of content in it.
- You can buy the two parts for a discounted price now (https://www.paradoxplaza.com/pillars-of-eternity-the-white-march-expansion-pass), so it's not going to cost you more than if PART1 and PART2 had been released together.