Customer Data Sync/ Custom Loyalty Program

What is the best way to sync customer sales from multiple platforms that do not easily integrate? Currently we use square and shopify and ZAPIER to connect the two. It is a bit clunky and occasionally fails.

Ideally we would build our own integration, or we may just switch to a platform that integrates.
We have also downloaded data and synced it manually but we would like to automate this process.