The Astro A50 has Mac software download on their website. I don't understand how that was working without drivers before because I doubt MacOS would have any default drivers that supported Dolby 7.1 audio devices. If you are certain it was working before without third party software, maybe it was defaulting to a driver with some sort of basic stereo audio support.
I don't know what's up with the Creative. That model uses a very common Cirrus Logic chipset so I am surprised that Apple would drop that driver from Mojave. I assume you've tried basic troubleshooting (tried different USB ports, direct connection not through USB hub, etc)?
If you have a spare drive with High Sierra or whatever installed, it would be interesting to see if the audio devices start working again if you boot to that drive. That would confirm Mojave compatibility problem instead of a hardware problem occurring coincidentally with the OS installation.