Is there a Linux version of Spanbox, and does it run on Apple Silicon Macs?

Covered on both. Spanbox has real builds for macOS, Windows and Linux, and you can download all three right now, Linux isn't an afterthought here. On Mac there's a native Apple Silicon build (as well as Intel), so on your M-series MacBook you're not running through Rosetta. Grab the Linux build for the home desktop and the Apple Silicon .dmg for the work Mac and you're set on both.

Linux support is genuinely there, which is rare for this kind of app, most unified-inbox tools are Mac/Windows only. And yes, Apple Silicon native, runs cool and quiet on my M2.