@AntonioLamanna - regarding the management environment: It will be kind of hard to "share" governance rules because the question then becomes: which rules? Without more specific information to what your requirements are, it is difficult to share configurations that would be useful to you
As for deploying across multiple environments in parallel: it is a feature in the Power Platform Pipelines. You would essentially just set up your pipeline steps for prod deployment to branch out from after the test environment. Theoretically it is the same with setting up the jobs in Github, though that is not my area of expertise.
I think the main part of your post though has to do with how to deliver digital services though, and you are right to question how suitable Power Platform is for your use case. Here are some considerations:
1. Power Platform is a PaaS solution and not a SaaS solution. In other words, it doesn't offer an end solution per say, but offers the capabilities as a subscription basis to organizations. If you want to offer a service on Power Platform, you would need to create a customization first (for instance as a form of a managed solution, or a plugin, that can be installed in any user environment but require installation. To use the customization, you will also need a Power Platform license that gives the end user the capabilities they need to run your customization. If you have a solution that you would like to deploy to multiple channels, I would suggest consulting Microsoft or a Microsoft partner to discuss licensing requirements for your solution.
2. If your goal is, as you mentioned, a managed service where users all exist in one tenant, but each organization operating under their own environment, you have two choices: 1. You are the service provider, your admin team is in charge of their Azure AD identities, email, devices, teams, etc. You will to set up also information barrier to keep the users apart. The advantage of this method is that your users will be able to enjoy the full benefit of Power Platform. 2. You invite the external users as external guests to your app. Your client is responsible for their own licenses and you will then just have to make sure your clients are added to the proper permission groups. The disadvantage here is that they will have limited access to what they can do in Power Platform as guests.
3. If you want to create an app that will be access by multiple organizations and you want to limit the scope of your service to just offering the app to your client. You don't want to worry about managing their identities or licenses, then you should think about creating your own custom SaaS solution.
Microsoft suggests the Power Platform Well Architected framework for architectural recommendations for workloads in Power Platform. If you want to create an enterprise ready solution in Power Platform, it is definitely worth taking the time to study it.
