Building my first Shopify App. What is your advice for me?

I'm a non-tech founder building my first Shopify app. I'm quite new to Shopify and would love it if you could help me clear out the fog. What are some of the key lessons you have learned building for Shopify?

• How long does it take to get approved by Shopify? (What are the criteria?)
• Do I need to set up a demo shop to get started?
• Should I partner with a freelancer / agency? (any recommendations?)
• What is the typical budget for building a Shopify app?
• How long did it take you from build to launch?

Looking forward to hear your thoughts :)