It would be wild if they open sourced it. Hell I would throw money their way if they did that. Theres a good game underneath all the MMO aspects they have been trying to push. Your right, they really need a way to self host it. One of the ways Coromon (does not need MMO) and Palworld (you can do just about anything) are better options in my opinion.
I don’t understand why they insist on the MMO aspect, people are only coming back for a little while but that wouldn’t be an issue if Temtem weren’t live-service.