I’m aware of the problems associated with using the official library as the resource for schematic design. But git makes it quite attractive due to naturally lightweight branches. Then, effectively there is no difference between using a copy of a git library snapshot (or as installed from a bundle package). But in this case I’d also prefer to add new symbols in a (global) library file that represents the family the symbol ‘belongs’ to. Regardless the question: contribute to upstream or not, the branching workflow is clearly preferable!
BTW: Interesting to take notice of visual diffs through a project, referred to on your Project and libary setup for sharing and collaboration (KiCad version 5), “Further reading” -> https://jnavila.github.io/plotkicadsch/.
In the case of footprints, merging is not such a problem because every footprint is in an individual file.