If you look at Sam's arc through ASoS and AFFC you see Sam becoming a lot tougher, at least mentally. He has lapses, for sure, but his cunningness in swinging the LCC vote for Jon, then getting to Oldtown via Braavos, finding out intel on Dany etc, show a marginally harder side to him. It's just that Samwise Gamgee was pretty tough to start off with (a life of physical grafting, albeit 'just' gardening, does make you slightly more prepared for endless trekking and hand-to-hand combat), whilst GRRM is showing us the 'toughening up' of Sam Tarly. I'd expect to see this continue in The Winds of Winter as Sam has to survive the machinations of the Citadel, plus whatever Jaqen and Sarella are up to.
Aegon IV a mental incompetent? No, not really. It was Maegor I who was actually a lunatic (and, it is pretty heavily hinted, was killed by his nephew or by his own Kingsguard to stop his insanity from continuing to destroy the realm in the midst of the civil war with the Faith Militant). Aegon IV was physically brave and a competent (if not outstanding) battle commander who also just happened to be cruel, vindictive, petty, violent and acted without thought for the consequence. That's much closer to Henry VIII. Robert has some of the same problems, in that he was a warrior without an enemy who became slovenly, but he didn't deliberately plunge the whole realm into chaos on a whim (as Aegon IV did when he legitimised his bastards and possibly hinted that Daeron II wasn't his son, paving the way for the Blackfyre Rebellion).
