When you start, services like Heroku, Firebase etc.. gives you a lot of power. Imho it's never a good idea. For example I have server rendered websites in go sdk (with the template engine) and other website in full node js stack (in my case react but the concept stays).
I don't really see a use case for mixed use since having two different languages slow you down.
- Problems to solve
- Poduct/Solution
- Growth/Marketing
- Pricing/Business Model
- Audience/Users
Then at the end have assingment to build a microstartups based on whats learnt!