I know the Onn remote will operate the TV's power and volume, but can the volume buttons be independently programmed to operate another device (at the very least, a soundbar, but I have an old Panasonic surround sound system I was hoping it might work with.) TIA!
The 4k box does let you add a sound bar to be controlled by the volume buttons, and Panasonic is one of the supported brands. Whether the IR code will work for your particular model you'll have to try and find out.
I've experimented with various setups and could config the remote to work the volume & input buttons on my LG tv and my Sony receiver. Can't see why it wouldn't work on your soundbar.
If the device has the IR codes for the soundbar it will be able to control it. Probably no way to know without trying it.