Well with 480 stamina per day of passive regeneration, 180 stamina from 12 gathers with 15 average per gather, you have total 660 stamina per day to spend on maps. With around 320+ stamina cap so level 90+ 2 FG would equal then a "bonus" day of farming. So for someone like me 5 FGs equal 2.5 days worth of "lost time". Or 1600 stamina that equals between 96k to 160k trophies (depending on ratio 60 trophies/stamina that I get now, or 100 trophies/stamina you should be able to get in phase two). With resets that phase two should add where full clears cost from 157.5k to 210k - its a nice help if you are lucky with spins, not so much if you need to nearly fully clear each reset.
EDIT:
Hmm. Event is up for around 13 days. Half of that is phase one with its 60 trophies stamina, and 660 stamina per day - around 40k trophies per day. Lets say phase one is 7 days long - 280k trophies. Phase two will then be 6 days long and should let us reach 66k trophies per day (660 stamina 100 trophies/stamina) - so close to 400k trophies. Totaling over whole event 680k trophies. Could probably add like 10% to factor in level ups - 750k. To fully clear we need 1.365M so we are 615k short. With 320 stamina cap and spending FG to farm more trophies on phase 2 we have 32k trophies per FG - a bit over 19 FG to get missing trophies. Question is if its worth using FGs to fully clear.
With 100 trophies per stamina in phase two, and per spin costs starting at 3150 - thats 31.5+ stamina per spin. If boxes will be half full of crap (seeds, coins, 3* girls), then with such high stamina per spin cost full clearing will simply be not worth it. Of course assuming you get red-reset thingy in few first spins, its use FGs case if you get that on literally last spin each reset.