Just looking now and GA have filed sale fares BKK-CGK/MDC upto 31 Mar 18, so i'd hold off for a bit as they will lower those fares.
Two things i'd add - most Lembah resorts pick up foc for the 12-14:00 arrivals into MDC - so maybe an overnight in CGK / DPS (FM7 in CGK is good value) may not be the worst thing.
Secondly, having gone muckdiving for the first time last year - can only say it's incredible! My friend runs a lil 8 room dive resort there, so if yours fills up happy to make a recommendation for an alternative