That's a really good summary of the problem!
I did look into things like harfbuzz a couple of years ago, but they're just ridiculously huge - adding support for it would double or triple the size of most binaries that people build!
But yes, adding some kind of i8ln layer would be the smart approach - so that it can use a simple back-end that only handles LTR text if you don't need anything beyond that, but being able to swap it out for a more capable back-end in apps where that's necessary. It's a huge task, but is on our radar!