We had a similar rotation on Chrome team for merges from WebKit (pre fork), and it was similarly a lot of work and clunky tooling!
A few times in my career (including this one) I have thought, "We are sure going to a lot of effort to maintain a modified copy of that code while also preserving our changes atop it as we sync, and this is exactly the kind of workflow that Git was designed to enable." Like, the Linux kernel dev workflow is all about different maintainers maintaining different branches and merging between them, and that is where Git comes from.
So in a setting other than Chrome I have tried out using Git to try to manage these sorts of situation. I have found in practice many engineers aren't comfortable enough with Git to have it end up helping them out tooling-wise. This is disappointing but also not too unexpected given Git's UI.
A few times in my career (including this one) I have thought, "We are sure going to a lot of effort to maintain a modified copy of that code while also preserving our changes atop it as we sync, and this is exactly the kind of workflow that Git was designed to enable." Like, the Linux kernel dev workflow is all about different maintainers maintaining different branches and merging between them, and that is where Git comes from.
So in a setting other than Chrome I have tried out using Git to try to manage these sorts of situation. I have found in practice many engineers aren't comfortable enough with Git to have it end up helping them out tooling-wise. This is disappointing but also not too unexpected given Git's UI.