so, with bigger fin (eg. 26 or 28) 6.0 works OK on K79
for bump & jump and freeride?
Correct but please bear in mind I still don't have a 26 or 28 cm crossover/freeride fin yet to prove or disprove it.
The crossover 24 cm fin felt a bit too small whereas a 30 cm freeride fin (from Kombat 86) felt a bit too large for my taste @65 kg with my NR 6.2 sail and the freeride footstrap's setup. Since you're heavier than me, I would strongly suggest you try several fin sizes before buying if you can.
IMHO : in my humble opinion.