Imo skip the open source license. You’re wanting to give this specific organization access to your work. If you follow the suggestions here and license it under creative commons or some other open source license then you’re also opening the door for others to use that work. It would be better to give the organization use of your work in its current form but otherwise reserve all rights for yourself, via a custom (non open source) license. This way in the future you can continue doing whatever you want with it, and maybe that means open sourcing when you’re ready to do so.

Ofc if you’re ready to open source now then go for it.