Hello,
I have an M4 Pro MacBook Pro, upgraded from a 2018 Intel system in January. I don't believe there are any differences in AirVPN's setup. There may be a prompt to modify your Mac's network setup to allow AirVPN as a VPN when prompted, unsure if High Sierra has that prompt.
macOS 15 privacy and security are stepped up significantly, so you'll run into more prompts and visits to System Settings' Privacy & Security tab in general. The OS expects notarized/signed apps and nothing that requires installing kernel extensions, which requires reduced boot security via Recovery Mode. Those are deprecated in favor of DriverKit extensions, but the Eddie client doesn't make use of this.
Eddie works great with macOS 15 and Apple Silicon, no different than Intel in my experience. The M1 version of Eddie works with any M chip, but you will still need Rosetta 2 installed, which is Apple's x86 to ARM translation layer. The core parts of Eddie are native, it's just the GUI that is still x86 until Visual Studio/Xamarin implements support for it. No biggie. You'll get a prompt to install Rosetta 2 systemwide with any app that still relies on x86 code, and once its installed, you're good to go, whether it be Eddie or another app. It's very performant: faster running a translated x86 app on ARM than actually running a x86 app on an Intel Mac.
WireGuard works fine, no issues on my end. There was apparently one WireGuard issue that was fixed with the last release in January.
Take care.