I don’t think that’s possible. As a sort of workaround you might be able to first backup all your other bookmarks on each device first, and then remove them. Then turn on sync and let it sync your keyword folder to all devices. Next, turn off bookmark sync again and then restore the backups.

Probably easier to just manually create the keyword folder its bookmarks though, since as a result you’ll just end up with a not-synchronized keyword folder, but only having a snapshot copy of it on each device.

From Firefox 113 onwards -moz-box related properties won’t work because the UI switched to using standard flexbox instead. So your -moz-box-ordinal-group: 2 won’t do anything. Use order: 2 instead.