Ask HN: How would you open source your SaaS?

I'm working on a SaaS MVP and the plan is to open source it so users can customize or integrate it using their own existing system.

My question is, should I open source just the frontend and have them connect to the backend using an API key (if they self-host) or should I open source the backend too?

I've gone back and forth on this for a while now and both sides have their pros and cons. Any guidance or help would be appreciated. Thank you.