Will we own the Filament customisations and IP?

Yes, completely. Source code ownership and IP transfer terms are agreed in writing before kick off, the NDA is signed before any project discussions, and the repository is yours from commit one. Custom Filament plugins, resources, fields, and widgets built for your project are yours, not white labelled or licensed back. Filament itself is MIT licensed and free, so there are no platform fees or licence multiplication concerns.