This is what I did in this order:
Feed Fewell 5/5 (Helps get synths done faster)
Safety Lever 5/5 (Keep impurities below 30% and the furnace will never explode)
Repair Furnace 5/5 (Useful for difficult synths)
Thwack Furnace 1/5 (Because Pressure Handle and Recycle Fewell are useless)
Don't bother with Thwack Furnace early on. When skilling up, who cares if it explodes, and when you are doing synths, keep impurities below 30% and you are fine. Pressure Handle only affects explosion damage and Recycle Fewell just gets some used fewell back, so they are both pretty useless.
This is the advice I give to anyone about synergy. Hope it helps.