And, to be honest, for whatever reason it has happened, they have been treated appallingly by Israel in recent years. I imagine if I was growing up there, I would hate Israelis and, probably, Jews by extension. [...] I visited Israel only once as a teenager in the early 80s, and it was definitely more harmonious on a day-to-day basis.
That is perhaps true for Israelis - not really for Palestinians, though! The awful treatment they have been suffering at the hands of Israel is not a recent development but goes right back to the 1940s - even before the establishment of the state itself!
Yes, the early 80s might have seemed relatively harmonious if you were living in Tel Aviv - they most certainly would have felt a lot less so if you were Palestinian. Most famously, of course, the decade was essentially "kicked-off" by the Israeli invasion of Lebanon, which led to several large-scale massacres specifically of Palestinian refugees (this is, literally, how former Israeli PM Arik Sharon earned the nickname "Butcher of Beirut"). Moreover, the 80s saw quite the concerted campaign to expel Palestinians from Jerusalem as well as a doubling-down on colonising the West Bank with more and more settlements and what is arguably best described as "consolidating and professionalising" the military occupation of the West Bank, East Jerusalem, and the Gaza Strip.
It is for these reasons that, in 1987, the first Palestinian intifada started as the largest grassroots collective effort of resistance against what - to Palestinians - felt less "harmonious" and more like slow strangulation.
FWIW, almost all Palestinians I know do not hate Jews as such (though Israelis are often referred to as "yahood" (Jews) in colloquial Arabic - though "zayooni" (zionists) seems to be gaining in popularity these days, too). They all absolutely despise Israel, though. As you say yourself: given their experience, that is somewhat to be expected.