I went through this same thought process - I'm 5'10 as well.
S3 is the size that 'fits' according to Specialized, and I tried both an S3 and S4. I could certainly ride an S3 but I prefer my bikes to be larger rather than smaller, so I went with an S4. Perfectly happy with that choice.
I'm looking into options for derestricting my EP8 powered bike but am not sure whether software or hardware is the way to go.
I looked at the ST Unlocker app previously, but that seemed to just offer changing location to US and so increasing from 25 to 32kmh - not sure if that is still the...