Events are global; Lua events even more so. So they do interact with everything regardless of surfaces. Surfaces in fact are nothing special except more chunks. Very little is actually seperate per-surface. The entirety of force-related anything is global, events are global, player stuff is global.
Factorio is not limited by cores; it's limited by memory access; cache and latency of getting data in and out of the CPU. The threading of belts reinforced this since updating them fully saturates the available cache and caching logic on even the fastest CPUs we have available to test. Running more things in parallel gives no improvements to speed at that point.Also one of the arguments in the beginning of this thread was, that normal CPUs don't have many cores, and only high end machines would profit. Now that even entry level CPUs like Ryzen 5600 have 6/12 cores/threads that argument won't hold for much longer, and since you're planning with a timescale of about a year, those will have quite some market share.