Since the advent of this convoy system (2.0?) there have been less than 5 cases where I was unable to get an SS. Half of those was due to me opting not to take an SS when offered during a time period where I know I might be plundered.

For the record, I get, on average, ~12 refreshes per day. Roughly 6-7 during the night session, and 2-4 during noon session (sometimes the full 6-7) plus the 2 free manual refreshes. So your result might be better or worse if you can get more or less than that amount of refresh.

The thing is, while I get SS when I want vast majority of time, I do have trouble getting all max value S. In fact, I only get 3 of those about 70% of the time. Every week, for a day or two, I end up being one short. If SS is guaranteed on every refresh, then there would be fewer S, and by extension, less max value S. So while I might gain 100 more coupon, over the course of the 6 months, I would be missing out maybe 5-10m coins. It's hard to say which is more valuable due to lack of easy conversion.

Of course, if all you ant is guaranteed SS on the two manual refresh, then that might work better. That should have relatively little impact on the coin income, maybe 1-2m only, probably worth giving up for 100 coupon.