You've already recognized what your best options are, so you're off to a good start.
If you've got a long enough time horizon that you'd like to stay out of bonds, then that only leaves you three funds:
FXSIX SPTN 500 INDEX INST 0.04%
FSEVX SPTN EXT MKT IDX ADV 0.07%
FSIVX SPTN INTL INDEX ADV 0.12%
Having to rebalance among three funds, once per year, is not too bad. I think you should give it a try and see what happens. Down the road, if you find that you're having difficulty rebalancing, then your idea of going 100% S&P would be fine.