I think I'm finally ready to release this project to get a bit more feedback. It contains pre-compiled 509.1.1 versions of the main swift-syntax modules for macOS, iOS simulator and device in a form that you can add to a project using macros to avoid excessive recompiles. It didn't quite turn out as I expected as you don't subscribe to it directly from a macro package but drag the repo onto your app project using macros so it takes the place of anything named swift-syntax in the macro packages you are importing.
In this way it acts as a firewall between your project and the 38,000 line swift-syntax source repo and hopefully will go a bit easier on Xcode. It also means macro developers don't have to subscribe to the binary repo directly which would create versioning nightmares. For details on how to adopt it consult the README of the project. For some reason your project needs to have built without the package override before you add it but perhaps even that will get solved eventually. Right at the end I discovered someone else had made a start on this back in November. This repo is very much the same idea with a few gnarly technical kinks related to sandboxing straightened out.
Ultimately the place for these binaries is either on swift.org or in a toolchain but I can understand they don't want to declare ABI stability on swift-syntax just yet. Hopefully this provides a solution (for Xcode) in the meantime. Feel free to raise any issues you come up against on the repo.