Do you know your landscape?
Do you know how many interfaces you have available on your SAP PO system? If not, why not start with that? There are a few solutions that can support you with that: UDO from Arianim and WHINT Interface Catalog (On-Premise).
Are there any other SAP Integration Suite developer tools I should know about?
SAP CPI helper – a must-have which extends the SAP Cloud Platform Integration with some useful features to improve usability.
Groovy IDE – groovy is the way to work with SAP IS so you can use this IDE to do it a professional way. Check out also other products from pizug.
Int4 Shield Lite – a free solution to test SAP PO to SAP IS migration and automate this migration.
Int4 Shield – an enterprise-level test automation solution for testing SAP integration scenarios (recommended by SAP and used by SAP)