If the opnsense interface on the WAN VLAN has a public routable IP address there shouldn’t be a problem with double NAT. Double NAT should only be a problem if they have a crappy ISP that’s using CGNAT.
Edit: never mind, I reread your comment. We’re saying the same thing essentially.
I like my Denon Heos setup: 2 TVs, home theater, receiver in my office connected to my computer and speakers in 7 other locations. Works great with Music Assistant, and doesn’t require a cloud connection. It can pull firmware updates if you want but I’ve blocked all Internet access for those devices with no loss of functionality.