They today open sourced their server which can now be selfhosted https://ente.io/blog/open-sourcing-our-server/
I would gladly pay $10-20/mo for Ente apps to use my own backend. Unfortunately I have around 8TB of photos so paying for a storage plan is out of the question.
It seems more likely this is only open source for audit/transparency purposes.
It’s absolutely their right to do this, but they should make it clear.
Addition of an option in-app is being discussed @ https://github.com/ente-io/ente/discussions/504