Advice 🙏🏻 non-KDP print-on-demand recommendations for niche series

I’ve completed two 3-book series (6 books total, 100 pages each, paperback). Four of these are in a highly specific niche that I haven't seen covered anywhere else, so I'm excited to get them out there.
Unfortunately, I’m stuck in a loop with Amazon KDP identity verification and need to move on to other platforms. I’m looking for an alternative Print-on-Demand (POD) service that handles printing and international shipping well.
Any advice on which handles "low content" or shorter paperbacks best?
Appreciate any advice. thanks