Syncing data between devices / different apple ids, passing the cost to the user

I'm researching ideas where I can sync data between different devices, owned by different people aka different apple ids.

I'm currently using sqlite and I really only need to sync transactional changes.

Ideally I don't want to have to store a full copy of each users database online.

I'm looking for an approach where the user has to pay for the service. I know I could setup some kind of subscription.

Can anyone shed any like on solutions that might work ?