There are lots of questions to your simple question but I do appreciate that using Content Porter (at least using the Content Porter GUI) may be very cumbersome. So, with that said - remember you can use the Import/Export services to remove GUI interaction if it suits the solution/processes etc.
Thinking very generically have you looked at the possibility of standing up the different CM instances wherever the need to go from a single database backup. It sounds like you could then selectively delete the publications (and related content) that you don't need for each site.
Caveat - this is super-dooper-high-level.
EDIT: In answer to the comment on 'shared infrastructure'...
It seemed that you intended to preclude a shared infrastructure with the statement "to move to a different entity/company having their own cloud and everything".
Maybe you can provide a tenanted system thus requiring only one SQL Server License but I'm not sure how you would best approach from the perspective of requiring either a multi-tenanted CM environment; if this is the path required then different CMs won't work but you're likely looking at different publications/Bllueprinting requirements.
It sounds like a fantastic challenge but not something to review in this forum (sounds more like a business/implementation , in general, challenge).