Actually, De-rated & Assumed Temperature (TFLEX) Rating are completely came from a different aspect of view in term of operation
๑ 1 Derate Good on Short RWY gives you more MTOW than Full Thrust but you can not apply Full thrust since Vmcg/Vmca = More MTOW = Company HAPPY
๒ 2 FLEX you can still apply TOGA at any phase if needed- contrasted to lateral handling difficulty. It will consumed more fuel then TOGA to reach specified climb gradient but It reduced EGT = Engine Life = Company Happy