What does this have to do with web development in particular? Regardless of the kind of software involved, if YOU fork, it's YOUR responsibility to merge upstream patches and fixes over time. If the various vendor-branded Chromia aren't doing it, then it's entirely on them. Upstream Chromium is open source, and the code for all of those fixes and patches is available in the open.
Don't bother with a LTE/patch-only branch, just "iterate iterate iterate" as the just user coming down the net connection will get your latest anyways...
Maintaining a fork as complex as Chromium is probably more work than these companies are willing/able to do.