Is compressed debug info a thing? It seems likely to compress well, and if it's rarely used then it might be a worthwhile thing to do?

It is: https://maskray.me/blog/2022-01-23-compressed-debug-sections

But the compression ratio isn't magical (approx. 1:0.25, for both zlib and zstd in the examples given). You'd probably still want to set aside debuginfo in separate files.