I've played all of them (west releases anyways), except for the new three that haven't gone on sale on steam yet (idk if thats Trails or Tales off-hand, love them both)
As for Trails in the sky, it uses the same world for part 1/2 but other than that is not repetitive as far as plot and other things, and they are both incredible games, maybe lacking in replay value due to the world mao being the same areas (mostly)
But the MC is hilarious almost always...never forget when she attacks oliver, best scene ever
I love the series to death...as well as Trail of Cold steel and "Tales Of", one side note on Tales Of:
The "Tales Of" has an active battle system, where as Trails does not, its a turn based RPG.
Part 3 turns into more of a Dungeon Crawler (to remove the same map I suppose lol)...and I wasn't too big on that one particularly...although the basis of the game is still the same and others might disagree.
Sorry this post was based on me thinking you wanted an opinion before purchase lmao.
And typed in a hurry since I was busy IRL, corrected whatever typos I caught...but i'm still busy.