A couple comments (mainly related to Flan bc only hero I finished all 3 difficulties with, but melee in general)
I think two-handed swords need to be totally rebalanced. Comparing a [P] Star Chaser and the set weapon Laevaeteinn (I have the full set bonus), I sacrifice 75% block chance (from Kourindou's shield), additional armour, and 7 sockets (3 from Star Chaser, 4 from shield), to I end up with only 2000 additional damage. Then I realized the 7 sockets make the difference in damage to be only around 500 or so bc additional stats. Not to mention two-handed swords have a way slower frame rate (I think about half as fast?) as one-handed swords. Since one-handers swing about 50% faster, I actually do more damage with a one-handed sword AS WELL as having extra survivability. I feel two-handers need to be far stronger in exchange for the lack of sockets (you made socketing far more important) and extra block chance. Maybe change two-handed weapons animations into one-handers to deal with the frames per attack gap?
As for Flandre's set specifically, Holy Fire aura and % to cast Enchant are some questionable bonuses. I can understand Enchant for Youmu builds wanting enchant or something, although Flandre builds don't need it (I think level 20 enchant overwrites my level 25 enchant buff, which makes it terrible actually) but Holy Fire cant even kill enemies on normal difficulty lol.
Also, melees require lifesteal to survive. With lifesteal being 33% effective in Hell mode, (I'll admit lifesteal is REALLY GOOD on lower difficulties) and enemies having a much lower drain effectiveness (lowering % life stolen even further), perhaps you would consider +x life on hit modifiers, or giving absurd amounts of lifesteal at higher levels.
Because of the lifesteal issue, Flan's Forbidden Games is kind of weak, especially with diminishing returns. It doesn't really accomplish what you were trying to have it do, which I assume is regenerating absurd amounts of life/mana against single targets. I'm better off chugging potions. And her Forbidden Fruit feels really... lackluster as a final skill. 25 points to absorb about 50 hp per fire attack is nice, esp after resistances, but it is conditional and I can't justify putting more than 1 point in it.
Pesco, some of your curses are really really annoying to fight against. As a melee, getting cursed for like -60% aspd means you stand there for a good 5-6 seconds in a mob of enemies. It also trolls hit-recovery frames too, so you can only sit there permastunned, spamming pots and hope you survive until the curse wears off. It literally is a 5-10 second permastun for killing an enemy.
Generally, melees sacrifice way too much defense for not enough power in return compared to ranged, (Amra actually did more damage than me from Normal all the way to Hell, he only gave up bc poison immunities while I had 5 different elemental attacks) I'll post when I think of more.