Ys games are on sale on GOG so it's a good as time as any to give them a whirl.
For the series as a whole, I think the most major flaw of Ys' gameplay is just how clear cut of a difference one level up can make. This comes into play when dealing with normal enemies where one level up is all the difference it can be between not being able to kill them to being able to do anything to them or straight up murder them. Bosses are fine though since at that point you're essentially dealing with a puzzle and reflexes.
It should be noted that Origin has three playable character arcs. You have to play all three to get the real ending. Each character are built differently from each other so how they go about the tower are quite different. I think this is where the argument of whether Oath or Origin is better comes from. I would say that Origin is more varied but the boss designs are less refined, I suppose? In Oath you only play as one character, Adol the Badass (essence of the whole Ys story). In turn Falcom was able to tune the whole game around him.
With that all said, they're still fantastic games. I do a yearly replay of them.
This should interesting. I wonder if Falcom are going to dive right into the lore stuff that they've been dancing around for so long. Like why is Adol is such a badass.
As for Trails, what FLD said about lore and character.
I also found SC to be harder. FC have a few things that you can do to easily overpower enemies and bosses. For example, Kloe's debuff move can stick to everything besides the last boss and it applies to both physical and arts damage. They're largely been rebalanced in SC. SC also rewards looking and planning ahead for gameplay and story which I don't recall in FC. 3rd seems to go even further and add in more gameplay elements.
Then again, if you don't like the foundation then anything built on it might not interest you.