Nicolas Longchamps

This is a vex snippet and not an hscript expression so you cannot use $F.
Use @Frame (or @Time so that your animation is not framerate dependant).

docs [www.sidefx.com]

Just think of your node network as a construction history that you never flush, and is also non-linear…
Make use of subnets to keep things sane, and you can always “freeze” your model by writing out as a bgeo and reading it back in. Or use the lock node which essentially freezes everything upstream from that node, and saves that geometry state in your hip file. This is interesting because it's like a history you can “unfreeze”.

Even with relative paths the materials would still be outside of the HDA, no? If you are referencing materials you need to pack them into the HDA in a material network, and use relative paths to the materials in there. That, or expose the path to the materials as parameters.