Should we also change this one or leave it explicitly as statically defined esp8285 ?
In the latter case, should we disable dynamic detection when esp8285 is selected by user ?

Either if we change board= to esp8285, right now it adds -DARDUINO_ESP8266_ESP01 when building with esp8285 as variant. Or, a custom flag inside of the variant header

And yes, there's no point in dynamic detection in the isFlashInterfacePin macro with specific variant.
Plus, only generic variant may add esp_is_8285() call, keeping the rest of variants with the original one.