I am at the same place you are in my FIRE journey. 60+ savings rate, all the low hanging optimization fruit plucked, any other optimization would yield smaller returns, but still worth it nonetheless. So now what, I have 4-7 years to go?
What I decided to do was to start optimizing my attitude along with my finances. Because we no longer eat out often, I decided to make eating at home more enjoyable, trying new recipes, finding really easy and delicious dishes that I don't really get bored off. Find new things to do around town that don't cost a lot of money, like more trips to the park, museums, community events, and hanging out with neighbors and family. Since I've mostly stopped going out to bars, movies, parties in the evenings. I've started reading more and learning things that I have always been curious about.
Basically with the free time I have, I've really started to experiment and try new things that really make me happy.