"Testing, testing, testing, to get started with automated ETL (ELT) testing have a look here: https://the.agilesql.club/etl-testing/
A re-envisioned IDE for SSIS packages, jump here
SSDT Dev Pack
A few helpers to make using SSDT slightly better, the killer feature for me is quickly deploying the proc or view you are looking at to a database - this means you can write code, quick deploy it and run your tests - red, green, refactor for T-SQL! jump to it
Code coverage reporting for T-SQL, ideally you would use it with tSQLt but works with anything, grab it from: here and remember code coverage tells you where you need more tests, it isn't a weapon to beat yourself or your employees with.