I still have no idea where I should place Mask of the Sun or The Lost City of Sand (is that Ys Expert? Or Dawn of Ys? So confuuuusing) in the timeline, or if they're Gaiden games since they were done by another studio.
Timeline-wise,
0th, Origin
1st, Ys I
2nd Ys II
3rd, Dawn of Ys (best), Mask of the Sun (+ a new thoery), Memories of Celceta (canon)
4th, Wanderers from Ys, Oath in Felghana (exact same story)
5th, Kefin, Lost City of Sand, Lost Kefin, Kingdom of Sand, Ys V Expert (Hard mode)
6th, Lacrimosa of Dana
7th, Ark of Napishtim
8th, SEVEN