Hi Lee,
Presently thinking 17/18 Sept, even that is a little late in the season. Based on a 6am start Snowdon, and roughly 14hrs (my best) to complete, you would not arrive Foel Fras untill after sunset, and that doesn`t include the walkout.
If you want to complete the challenge this year, you might want to consider a reschedule.
Dave
P.S. also possible for me midweek 12/16 Sept, on holiday !