You could select them all and do the Union, but that would unite them all as one combined path, which is obviously not what you want.
If you want each as its own separate combined path, then I think you will need to do them separately. Perhaps use the shortcut Ctrl++ to do the Union to speed things up a litte. Note that on most US/UK QWERTY keyboards that would actually be Ctrl+Shift+=
It is possible to automate Inkscape by coding, but I've no expertise in that field. I don't even know if it would be possible for this specific case. Sorry.