What are the dynamics of the group ride? Is there a decisive section (long flat section, 2 min climb, etc) that makes the difference?
For me, my group ride is a mix of long flat stretches, then a few sharp hills and a final long section of 2% grade. Like you, I tailored my training to put the hurt on the group, focussing on the hills to cause the most damage. It worked, I was regularly well ahead of people on those points. What I failed to recognize (silly thinking back) is that there is a regrouping point after these sections, so even though I was essentially first on these climbs, it wasn’t crushing the group like I’d planned. When I thought about it some more, I realized that the early long flat stretches can cause pain, but most decent riders can just sit in and keep up. The hills you can drop people, but it’s not permanent damage because of the regrouping point. It’s the long 2% climb (15 mins or so) after the group is already fatigued, that really blows the group apart. Now I can do my turns in the flat sections, finish with the lead people on the climbs without going all in, but then really turn the screws on the long final stretch.
I did SSB and then Sustained Power Build to give me much more ability to hold an uncomfortable pace on the front for long stretches. It worked fantastically well. I found I didn’t need to do Speciality (Climbing Road Race or 40KTT would have been sensible) to deepen this capability. Just repeating the Base and Build phases is enough to keep up a meaningful FTP difference over most of the pack.