It sounds like you want a few more riders.

Are your rides posted on a calendar or an email list? Make your "D" group a separate ride, not just the "D" group--give the ride it's own name. Include both your fiancee's first name and your first name as ride leaders, having a woman ride leader will help. Mention that you wait at the top and the bottom of the hills, with nobody left behind. This may bring out some riders that felt that the usual rides were just way too much to handle.

That "10-14" average speed is way too broad a range. Choose something more definite, like "Flat road speeds of 14-15 mph; hills and stops will make the whole ride average about 12-13 mph", and post the ride distance too. (New riders get in over their heads when they only see average speeds posted.) I've led rides that ended up being faster than the posted speeds, because everyone who showed seemed comfortable going that fast. But keep an eye on the last few riders and back off to the posted speeds if needed.

Make sure you don't lose any riders. (It's sooo easy for groups to get spread out!) Either count them at every turn onto a different road if the group is under 10 riders, or ask/appoint one of the riders that tends to be near the back to be the sweeper, and wait until they roll up to the group and get a chance to take a short break before you take off again. The sweeper's role is to make sure that nobody gets behind them.

(I've led some fairly slow group rides. I do manage to get some sprints in because I'll drop back to cruise with the last few riders, then have to blast up to the front to keep riders at the front from missing the next turn. It's interesting to ride slow, take in the sights and sounds, instead of watching the riders right ahead in the pack all the time.)