Weird, I have a homepod (mini) also. In fact I hardly have any Apple stuff anymore, I only got homepods because it was the most privacy-friendly option out of the big three. I just have an iPad from work which I used to set them up. And most of my automation goes through Home Assistant anyway.

When I give a double command Siri literally tells me she can't do two things at the same time and I have to present them as two separate commands.

Perhaps it's because I have it set to UK English? Perhaps it's smarter in US English. I'll have to try that.