I found a way to (essentially) do this. I am having trouble with the Japanese calendar as of iOS 18.1 and I needed a similar solution. While we can't change what happens with [NSCalendar currentCalendar], I found a way to simply not use it. I made an NSCalendar category and added this:

+ (NSCalendar *)adjustedCalendar {
    
    NSCalendar *cal = [NSCalendar currentCalendar];
    
    if (cal.calendarIdentifier == NSCalendarIdentifierJapanese) {
        cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    }
    
    return cal;
}

Instead of calling currentCalendar, I call adjustedCalendar and it always gives me a Gregorian one.