Good work on the work on the workaround. I want to share my workaround for this situation also. Basically,
I create an MP3 file to send to the HomePod as if I were sending a music file. This bypasses the 10s timeout because there’s no processing done while the HomePod waits. The upside is that I haven’t found a limit to MP3 file size, so theoretically you could have a 10 minute long notification spoken over your AirPods.