Hello @faruque5698

I read multiple questions and for each part will write down a short list, feel free to ask if you have more questions :)

Most parts aren't specific for a SAAS

Maintainability

• Create and document the code structure of your application.
  • Validate your architecture with tools like Deptrac
• Validate your code style
• Have automated tests for your code.
• Use static analyzers like PHPStan, PHPMD
• Run all checks in your CI
• Document your decisions.
• Do not create custom options for a specific client.
• Be consistent.

Efficient collaboration

• Use version control and have clear commit messages.
• Use a CI
• Write documentation
• Communicate a lot

Scalability

• Put everything that doesn't need to be done directly on a queue or at least make it possible.
• Only load the needed data. (Think about only the needed records but also only the needed fields)
• Think about your database design, check your queries and let your database do the calculations.
• Check your logs / use observability to monitor for potential problems.

And besides al enjoy your project :)